DXF export: an open path of one straight segment is written as a LINE

An open path with a single straight segment was written as a two-vertex
LWPOLYLINE (POLYLINE before R14). It is now a LINE entity from its start
to its end point. A stroke with a weight in R12, R13 and R14 keeps its
polyline, since a LINE has no width. Closed paths and paths with more
than one segment are unchanged.

Part of #1299.

docs/mcp.md

@@ -3832,7 +3832,8 @@ without its formatting, taking the style at the caret.
 `document.exportDxf` (also `document.export` / `export` with format `dxf`) writes a CAD drawing, ASCII DXF R12 to 2018
 (`version`: `R12`, `R13`, `R14`, `2000`, `2004`, `2007`, `2010`, `2013`, `2018`; default `2018`). Each layer becomes a
 DXF layer (hidden layers switched off, locked ones locked, non-printing ones not plotted; template layers left out),
-straight paths become polylines, curved ones cubic splines through every anchor, fills their outlines followed by
+straight paths become polylines (an open path of one straight segment becomes a line, except for a stroke weight
+before 2000), curved ones cubic splines through every anchor, fills their outlines followed by
 solid hatches (laser and cutter software reads the outlines and skips hatches; R12 has no hatches: the outlines
 alone), strokes lines with their lineweight and a linetype per dash pattern, and placed images image entities linked
 to PNG or JPEG files (`rasterFormat`) written next to the drawing (`linked` in the result). Coordinates are y up

docs/target-app-parity.md

@@ -1041,7 +1041,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   - EPS and PostScript import keeps a CMYK image's ink amounts, as PDF import does: the samples of `image` and `colorimage` in a CMYK color space, and of a CMYK JPEG read through `DCTDecode`, come in as a CMYK TIFF, read through the image's Decode array, and the pixels a `MaskColor` or `ImageType 3` mask hides become a grayscale opacity mask (`crates/eps/src/import/tests_images.rs`). Such images came in as RGB PNGs.
   - EPS and PostScript import reads image data through `CCITTFaxDecode`: Group 4 (ITU-T T.6) and Group 3 one-dimensional and mixed (ITU-T T.4) data, with the filter's `K`, `Columns`, `Rows`, `EndOfBlock`, `EndOfLine`, `EncodedByteAlign` and `BlackIs1` parameters, decodes to 1-bit rows for `image` and `imagemask`. Damaged data keeps the rows decoded before it (damage in the first row is a PostScript `ioerror`), and parameters out of range or past the 256 MiB limit on decoded data are refused (`crates/eps/src/import/tests_images.rs`). Reading such a file stopped at an error naming the filter, and the file opened from a fallback, such as its preview or the art drawn before the image, or did not open.
   - EPS and PostScript `.ai` import undoes the TIFF predictor (`/Predictor 2`) of `FlateDecode` and `LZWDecode` data at 1, 2, 4 and 16 bits a component as well as 8: each component adds the same component of the pixel to its left within its bit width, 16-bit components add as big-endian numbers, and each row starts again (ISO 32000-1 §7.4.4.4; `crates/eps/src/import/tests_images.rs`, `crates/engine/src/cmd/fileio/tests_epsimport.rs`). At those depths the image drew the stored differences instead of its samples.
-  - DXF control-point SPLINE import honors its closed flag (#1299 follow-up after #1320 fixed export), including rational curves; open curves whose endpoints meet stay open. Own-fixture regressions cover controls/knots, fit-point fallback and all export versions (`crates/cad/src/import/tests.rs`, `crates/cad/src/tests.rs`); native save/read checked. Circular cubic paths export as native CIRCLE/ARC where stroke semantics permit; R12 weighted strokes remain POLYLINE, and a non-circular ellipse keeps the existing fallback (`crates/cad/src/tests.rs`); CNC preset and VCarve QA remain pending.
+  - DXF control-point SPLINE import honors its closed flag (#1299 follow-up after #1320 fixed export), including rational curves; open curves whose endpoints meet stay open. Own-fixture regressions cover controls/knots, fit-point fallback and all export versions (`crates/cad/src/import/tests.rs`, `crates/cad/src/tests.rs`); native save/read checked. Circular cubic paths export as native CIRCLE/ARC where stroke semantics permit; R12 weighted strokes remain POLYLINE, and a non-circular ellipse keeps the existing fallback (`crates/cad/src/tests.rs`); an open path of one straight segment exports as a LINE (R12 to R14 strokes with a weight stay polylines, which carry the width) and reads back as the same segment (`crates/cad/src/tests.rs`, `crates/cad/src/import/tests.rs`); CNC preset and VCarve QA remain pending.
   - Affinity documents in (AF.1–AF.4): `.af` (Affinity 3) and `.afdesign`/`.afphoto`/`.afpub` (Affinity 1 and 2, container versions 8–12) open and place as editable art: layers and sublayers, groups (pass-through or isolated), artboards (with their names and backgrounds) and Publisher pages side by side, curves with live corners, parametric shapes (rectangles with corner radii, ellipses, polygons, stars, pies, triangles, trapezoids), compound shapes, solid/linear/elliptical gradient fills and strokes (weight, dashes, alignment, scale-with-object), clipping by nesting, vector and pixel masks, artistic and frame text with fonts, sizes, tracking, leading and alignment, placed images (their original JPEG/PNG) and pixel layers (8/16-bit RGB, CMYK), opacity, visibility, locking and blend modes, at the document's resolution in points. Effects, adjustments, brushes and pressure profiles, transparency gradients, master pages, symbol instances (imported as their cached picture), special shapes (cloud, heart, cog…, imported as ellipses) and Affinity-only blend modes are named in the import warning. A file whose native data can't be read opens as its embedded preview with the reason, and that preview can't be placed. Save never writes back over the Affinity file and there is no Affinity export. AF.5 (from DesignCraft's import, storytold/designcraft#291) adds master pages (on a locked layer behind the pages that show them), tables (area type per cell and a line per edge), each paragraph's alignment, indents, spacing and leading, horizontal text scale, outlined text, frame text sized by its text scale, drop shadows and outer glows as live effects (inner shadows as inner glows), outline effects as strokes, colour and gradient overlays baked into the paint, embedded Affinity documents as editable content (their artboards don't become artboards), pixel layers over 64 MP at a reduced resolution and HEIC photos from their stored pixels; automatic leading (size plus the font's line gap) fitted on thumbnails; underlines and strikethrough; strokes whose style is None (every shape stores one) left out, which also brings 6 public files closer to their thumbnails; line breaks inside a paragraph; table cell text centred as Affinity centres it, also where it is taller than its cell; CMYK pixel layers converted through the document's embedded CMYK profile (relative colorimetric with black-point compensation, as each layer's colour settings name), or kept as CMYK images when there is none; outline effects on stroked shapes around their stroke; artistic text whose paragraphs align differently aligned within its longest line, a frame's first baseline at cap height, and frames whose last lines' descenders hang out made tall enough to show them (all measured on a local document's own PDF export: baselines match to 0.1 pt); a local 56-artboard document's mean thumbnail difference went from 16.2 to 11.4, and its pages' mean difference from the owner's PDF export from 12.9 to 9.3. AF.4 adds Affinity 3 artboard recognition and actual-outline clipping, source-backed JPEG/PNG raster tiles, Unicode text-run recovery, stored fallback/exact installed fonts, OpenType feature flags and compound-geometry limits. 51 public CC0/MIT/Apache-2.0 files (33 current `.af`), pinned and sha256-verified (`cargo xtask corpus --affinity`), test native retention and every-board native/SVG/PDF/PSD output for current files; selected files compare their saved thumbnails. No Affinity reopen or independent paired PSD oracle was available; [source audit and complex-font gaps](affinity-validation.md); [scope, provenance and limits](../crates/affinity/README.md).
   - Placed documents: File › Place of a `.vectorcraft` file links to it by default (`file.place`'s `link`): one locked object showing an artboard of the document (or its art's bounds), read the way placing it without Link reads it, its symbols, patterns and images kept apart from the document's; without Link (or from bytes, not a file) it is an editable copy as before. It stays vectors in every output (SVG, PDF, EPS, EMF, DXF), and so do placed documents inside it, read from their own files up to 8 levels deep (documents that place each other stop there). On screen they draw from bitmaps cached per file and zoom step, shared by every copy and made in the background (a placeholder until ready), so a sheet of them redraws in about a millisecond; exports and prints draw them exactly. Like linked images, a save keeps only a preview of each (JPEG when opaque) unless Include Linked Files; output, and zooming past the preview, read the file again, falling back to the preview with a warning when it is gone or changed. Saves for older versions write them as plain art. Links check, update, relink, Update Links on open, Link Info (the artboard's size), Placement Options, Break Link (also Embed, Object › Expand: the editable copy placing without Link gives, its resources joining the document), Edit Original (the document in a new tab) and the Links panel cover them; saving a document updates the open documents that place it. The native format (v4) keeps data of 8 KB or more after the JSON instead of base64 in it.
   - Placed documents' bitmaps on screen: when a bitmap made in the background is ready, the desktop app renders the canvas again only where the objects that show its file are, when the frame shown was rendered on the UI thread and that part is at most half the canvas. Bitmaps drawn or made since a frame began are kept for that frame past the 512 MiB budget, up to 2 GiB, so a view showing more than the budget does not make them again in every frame. A bitmap that would take them past 2 GiB is not kept, and its file draws exactly at that zoom. A bitmap that can't be made (the file can't be read and there is no preview) shows the light placeholder and is not asked for again. Each ready bitmap used to render the whole canvas again, and bitmaps past the budget and bitmaps that could not be made were asked for again in every frame (`crates/render/src/tests/placed_document.rs`).