Fix #918: PSD and PSB files open and place as their merged image

File › Place and File › Open read PSD and PSB files: the merged image
every such document carries after its layers, which is what a linked PSD
shows. A placed PSD links like any image, so the Links panel updates and
relinks it, and it is sized by its ResolutionInfo resource.

The reader is written from the public file format specification. It
reads Bitmap, Grayscale, Duotone, Indexed, RGB, CMYK (through the colour
settings' CMYK) and Lab, at 1, 8, 16 and 32 bits per channel (32-bit
values are linear), raw, PackBits or zipped with or without prediction.
A document with transparency stores its merged image on white with the
alpha in the first extra channel. It comes back with straight colours,
also for 16- and 32-bit files, which say so in their Lr16/Lr32 block.
Multichannel files are refused with a message. The file is untrusted:
every read is bounds-checked and the image's size is checked against
the decode budget before anything is allocated.

PSB is a read-only format of its own, so exports never write a PSD under
a .psb name. The desktop packaging still doesn't associate either
extension with VectorCraft (UNASSOCIATED_EXTS): they belong to raster
editors. Layers (import options) are left for later.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/mcp.md

@@ -275,7 +275,7 @@ objects' fills or strokes differ (`fillMixed` / `strokeMixed`, drawn as a "?" pr
 | `invoke_menu` | `{command, params?}` | Invokes a menu item by command id. Includes UI commands such as `view.*` and `window.*` in remote mode. |
 | `open_panel` | `{panel}` | Remote only. `panel` is a panel id (`layers`, `swatches`, `colorGuide`, …, as `window.panel` takes) or its display label (`"Color Guide"`), in any case. |
 | `screenshot` | `{path?, scale?, artboard?, window?}` | Returns MCP image content (`image/png`, base64) plus a text block. Renders the artboard; `window:true` captures the app window (remote only). With `path` the backend (the headless session or the app) writes the PNG there too. |
-| `open_file` | `{path}` | Opens any readable file as a new active document: `.vectorcraft`/`.drawcraft`, `.vctemplate`, `.svg`/`.svgz`, `.pdf`/`.ai`, `.ait`, `.eps`, `.dxf`, `.emf`, `.wmf`, PNG, JPEG, GIF, WebP, TIFF, BMP (an image opens as a document of its pixel size). Templates (`.vctemplate`, `.ait`) open as a new untitled document. PDF, `.ai` and SVG files saved with Preserve Editing reopen as the document they carry. The reply is `document.open`'s: its `warnings` say what didn't come in as it was (an EPS whose PostScript can't be read opens as its preview, and the warning names the PostScript error, the operator and the procedure). `run_command document.formats` lists the formats. |
+| `open_file` | `{path}` | Opens any readable file as a new active document: `.vectorcraft`/`.drawcraft`, `.vctemplate`, `.svg`/`.svgz`, `.pdf`/`.ai`, `.ait`, `.eps`, `.dxf`, `.emf`, `.wmf`, PNG, JPEG, GIF, WebP, TIFF, BMP, PSD/PSB (an image opens as a document of its pixel size; a Photoshop file as its merged image). Templates (`.vctemplate`, `.ait`) open as a new untitled document. PDF, `.ai` and SVG files saved with Preserve Editing reopen as the document they carry. The reply is `document.open`'s: its `warnings` say what didn't come in as it was (an EPS whose PostScript can't be read opens as its preview, and the warning names the PostScript error, the operator and the procedure). `run_command document.formats` lists the formats. |
 | `save_file` | `{path?}` | Runs `document.save`: the document's own file in its own format (native `.vectorcraft` unless it was opened from or saved as SVG, PDF or a restorable `.ai`; then `warnings` say what that format loses). A path's extension picks the format (`.vectorcraft`, `.vctemplate`, `.pdf`, `.svg`, `.svgz`, `.ai`: a PDF carrying the native document, which reopens editable). |
 | `export` | `{path?, format?, scale?, artboard?, range?, selection?, outlineText?, options?}` | `svg`, `svgz`, `pdf`, `eps`, `dxf`, `emf`, `wmf`, `png`, `jpg`, `webp`, `gif`, `png8` (an indexed `.png`), `tiff`, `bmp`, `tga`, `psd` (layered), `txt` (the document's text), `vectorcraft` or `template` (a native template). The tool's `format` enum and `document.formats` list them, generated from the engine's format table. When `format` is omitted, it comes from the path's extension. PDF writes one page per artboard: all of them, or `artboard` (0-based) / `range` (`"1-3, 5"`, 1-based); the other formats write one artboard. `options` carries more format options (e.g. `{"quality": 80}` for JPEG). `selection: true` exports the selected objects cropped to their bounds (the reply adds their `bounds`, and reports `format` and the encoder's `warnings` as a whole-document export does); `outlineText: true` writes SVG text as paths. Template layers are left out, live effects are kept, and exporting `vectorcraft` never changes the document's path. Without `path` the bytes come back as `dataBase64`. Both backends run the same `document.export` call. |
 | `add_text` | `{text, x?, y?, width?, height?, path?, mode?, pathEffect?, size?, font?, color?}` | Point type at (x, y); area type with `width`/`height`; or `path` + `mode` (`area`/`onPath`) to flow text in or along a path, with `pathEffect` (`rainbow`, `skew`, `3dRibbon`, `stairStep`, `gravity`). |
@@ -1690,7 +1690,7 @@ passed straight back to `file.new`. Print presets and sizes without `units` star
 
 `file.place` puts another file's art into the active document as one undo step without touching the clipboard:
 a raster image at 100% of its physical size (the resolution its file declares, else 72 ppi; linked to its `path`
-unless `link: false`), an SVG as one group, a PDF/.ai page as one clipped group, with the images, symbols, patterns
+unless `link: false`; a Photoshop document as its merged image), an SVG as one group, a PDF/.ai page as one clipped group, with the images, symbols, patterns
 and swatches it uses. A VectorCraft document read from `path` is a placed document: one locked object showing its
 artboard `page` (or, with `crop: "bounding"`, its art's bounds), linked to the file and read again when it changes
 (see Linked images), and vectors in every output; with `link: false` (or from `dataBase64`) it is an editable copy
@@ -2289,7 +2289,13 @@ show) is the flat render. `maxEditability: true` turns layers and sublayers into
 named as the Layers panel names it (text objects by their text); layers with a clipping mask, an opacity mask, an
 appearance of their own or knockout stay one pixel layer. Hidden layers and objects are left out unless
 `hiddenLayers: true` writes them as hidden layers. `layers: false` writes one flat image, on white where nothing is
-drawn. Files are at most 30000 pixels a side; PSD files don't open in VectorCraft.
+drawn. Files are at most 30000 pixels a side.
+
+Photoshop documents (`.psd`, and `.psb`, the large document format) open and place as their merged image, the
+flattened picture every Photoshop file carries (layers aren't read): Bitmap, Grayscale, Duotone, Indexed, RGB, CMYK
+(converted with the colour settings' CMYK) and Lab, at 1, 8, 16 and 32 bits per channel, sized by their resolution.
+A document with transparency keeps it. `file.place {path}` links the file like any image (`links.update` reads it
+again when it changes); Multichannel files are refused.
 
 ```json
 {"name":"export","arguments":{"path":"/tmp/poster.psd","options":{"ppi":300,"maxEditability":true}}}