M6: experimental .fig export

An experimental, best-effort `.fig` export, so work done in FigCraft can be handed back as a `.fig` file. It is written from the same public format write-ups the importer uses and is the inverse of the importer's mapping. Only round trips through FigCraft's own importer are verified; other apps may not open every file.

- **Encoder** (`crates/format/src/fig/kiwi.rs`): the kiwi encoder that test fixtures used is now shipped code. It never panics and checks every value against the schema: unknown fields or enum names, missing struct fields, wrong types, out-of-range numbers and non-finite floats are errors, and NULs inside strings are dropped. Varints and floats are written exactly the way the decoder reads them. `Schema::parse` reads the compact text schema form, and the importer's tests use it too.
- **Writer** (`crates/format/src/fig/export.rs`): writes the zip (`canvas.fig`, `meta.json`, `thumbnail.png`, `images/<hash>`). `canvas.fig` holds the fig-kiwi header, then a raw-deflate binary schema and a raw-deflate message (flate2's pure-Rust backend, already in the tree; no new dependencies). The schema covers only the fields we write. The canvas, images, blobs, node count and nesting depth are capped, and floats are made finite before they are written.
- **Mapping:** pages; frames, groups, sections, rectangles, ellipses (arcs), polygons, stars, lines and vectors (vector network blobs with per-point radius, mirroring and caps, plus fill outlines); text with styled runs (UTF-16 indexed) and list types; slices; boolean groups; solid, gradient and image paints (image bytes included); strokes; effects; corners; constraints; masks; auto layout, including grids with tracks and cells; child layout; layout grids; export settings; components, component sets and their properties. Instances are written as their main plus the fields their layers change, by id path. This covers nested instances and swaps, and fields the main sets but the instance clears. Prototype interactions (triggers, navigation and overlay actions, transitions, easing), scrolling and flow starts are included. Everything skipped is reported in a warnings list: style links, variables, comments, Dev Mode data, plugin data, guides, slots, brush and width-profile strokes, pattern fills, image adjustments, variable and conditional actions, and more.
- **Import fixes found by the round trips** (`map.rs`): ellipse arcs, boolean operation types and component set descriptions are now read. Durations are read as whole milliseconds, because seconds stored as 32-bit floats used to truncate by 1 ms.
- **File writes:** a failed write no longer leaves its `.tmp-figcraft` file behind. The junk-parameter sweep found this.
- **Commands and UI:**
  - `file.export {format: "fig"}`, or any path ending in `.fig`, writes the whole file and returns `warnings`. Without a path it returns base64 `data`.
  - File > Export... has a new "The whole file" section with a ".fig (experimental)" row. A note under it says other apps may not open every exported file. After export, a toast reports how many kinds of content were left out.
  - `figcraft-cli export --format fig` (or `-o x.fig`) and `convert` to `.fig` print the warnings as `note:` lines.
  - MCP's `export` accepts `format: "fig"` and passes the warnings through.
  - New UI strings have English and Spanish entries.
- **Docs:** file-format-parity.md has a new export section covering what is written, what is skipped, and that only round trips through our own importer are verified. Also updated: mcp.md, cli.md, gaps.md (gap 1), target-app-parity.md and ROADMAP.md. Grading is kept low. Import/export goes to 86%/58% and file formats to 60%. The file-exchange discount stays at x0.85, because nothing shows that another app opens these files.

docs/cli.md

@@ -31,12 +31,16 @@ page 1 "Screens": 380 nodes, 4 top-level
   …
 ```
 
-### `export <file> -o <out> [--format png|jpg|svg|webp|pdf] [--scale N] [--page NAME] [--node ID|NAME] [--bleed LENGTH] [--crop-marks] [--bleed-marks] [--cmyk]`
+### `export <file> -o <out> [--format png|jpg|svg|webp|pdf|fig] [--scale N] [--page NAME] [--node ID|NAME] [--bleed LENGTH] [--crop-marks] [--bleed-marks] [--cmyk]`
 
 - `<out>` is a directory: one file per top-level frame of the page (default the first page), named
   after the frame (`Home.png`, `Settings.png`…); with no frames, one file per top-level layer.
 - `<out>` has an image extension: one file with the given `--node`, or the whole page composited.
 - `--node` takes a layer id (`12` or `0:12`) or a layer name.
+- `--format fig` (or an `<out>` ending in `.fig`) writes the whole file as `.fig`, experimental:
+  written from public format notes and checked only by opening it again in FigCraft; other apps
+  may not open every file. What was left out (shared style links, variables, comments, Dev Mode
+  data…) is printed as `note:` lines.
 - The format comes from `--format`, else `<out>`'s extension, else png. `--scale` (default 1, max 64)
   applies to raster formats.
 - A `.pdf` `<out>` without `--node` holds the page's top-level frames, one PDF page each in reading
@@ -56,6 +60,7 @@ figcraft-cli export app.figcraft -o exports/ --scale 2
 figcraft-cli export app.figcraft -o hero.svg --node "Hero"
 figcraft-cli export app.figcraft -o screens.pdf --page "Screens"
 figcraft-cli export app.figcraft -o poster.pdf --node "Poster" --bleed 3mm --crop-marks --cmyk
+figcraft-cli export app.figcraft -o handoff.fig
 ```
 
 ### `convert <in> <out>`
@@ -64,7 +69,7 @@ By extension. Inputs: `.figcraft`, `.json` (FigCraft or REST API file JSON), `.f
 shapes, text, gradients, masks and effects; `info` lists what it left out as `note:` lines), `.png/.jpg/.webp/.gif/.tif/.bmp`
 (an image layer). Outputs: `.figcraft`, `.json` (FigCraft JSON, pretty-printed), `.svg`,
 `.png/.jpg/.webp` (the current page rendered at 1x with its background), `.pdf` (the current page's
-frames, one page each).
+frames, one page each), `.fig` (the whole file, experimental, as `export --format fig`).
 
 ### `render <file> -o <out.png> [--scale N] [--page NAME]`