Commands: a color param with an alpha part is refused instead of losing its alpha

Squashed from #5603 onto main.

docs/mcp.md

@@ -385,6 +385,10 @@ leaves it out when `ids` list it together with a group that holds it.
 
 Paint values (`fill`, `stroke`) accept `"#rrggbb"`, `"none"`, `[r,g,b]` (0..1), `{"c","m","y","k"}`, `{"gray"}`,
 or a full `paint.setFill` params object (`{"gradient": …}`, `{"swatch": "name"}`).
+A color read this way (a fill or stroke color, a swatch's, an artboard's or new type's color, and the other colors
+commands read in these forms) that is given as a hex color with an alpha part (`#rrggbbaa` or `#rgba`, with or without
+the `#`) is an error and changes nothing, as VectorCraft's own rule. The `color` of Drop Shadow, Inner Glow, Outer Glow
+and Shift to Color is read differently and takes `#rrggbbaa`, as described below.
 For `draw_shape` and `draw_path`, omitted paint fields inherit the current [new-art appearance](#new-art). Both
 also style the new object in the same call: `dash` (lengths, or null for solid), `cap`
 (butt|round|square) and `join` (miter|round|bevel) go to `stroke.set` with `strokeWidth`; `opacity` (0–100 %)
@@ -701,7 +705,7 @@ its effects paint, the effects of groups, type and images included. `artboard.fi
 way.
 
 Raster exports take more options in `options`: `ppi` (72, 150, 300…; stored in the file, except in GIF, WebP and
-Targa), `background` (`transparent`, `white`, `black` or `"#rrggbb"`), `antiAlias` (`none`, `art`, `type`: text
+Targa), `background` (`transparent`, `white`, `black` or `"#rrggbb"`; `#rrggbbaa` and `#rgba` are refused), `antiAlias` (`none`, `art`, `type`: text
 snapped to pixels) and, for PNG, `interlaced`. `width` or `height` sets the size of each file in pixels: each artboard
 (with `useArtboards`, each file), `rect` or art bounds gets the scale that makes it that wide or high, within 0.01–64
 pixels per point, and the file stores the resolution that results, as with `ppi`. `width` wins over `height`, and
@@ -3952,7 +3956,7 @@ still rename it). A Shift-click or Shift-drag on a locked artboard selects or dr
 document summary (`document.inspect`) adds `locked: true` to a locked artboard.
 
 An artboard's background color (Properties › Artboards › Fill, Artboard Options' Background Color): `artboard.setProps
-{index, color}` sets it, with a color as the paint commands take it (`"#rrggbb"`, `{c, m, y, k}`, …), and
+{index, color}` sets it, with a color as the paint commands take it (`"#rrggbb"`, `{c, m, y, k}`, …; `#rrggbbaa` and `#rgba` are refused), and
 `artboard.setProps {index, color: null}` removes it, each in one undo step; `artboard.new {color}` makes an artboard
 with one. The canvas draws that artboard's paper in
 the color. With Simulate Colored Paper on, it multiplies the color with the paper color, each sRGB channel as
@@ -4396,7 +4400,7 @@ selects, and the selected objects that are object slices. `slice.list` reports b
 `object.slice.fromGuides` (the grid the ruler guides cut the artboards into), `object.slice.fromSelection`,
 `object.slice.duplicate {dx?, dy?}`, `object.slice.combine`, `object.slice.divide {rows? | rowHeight?, columns? |
 columnWidth?}`, `object.slice.deleteAll`, `object.slice.options {kind?: image|noImage|htmlText, name?, url?, target?,
-message?, alt?, text?, background?: ""|matte|#rrggbb, hAlign?, vAlign?}` and `object.slice.clipToArtboard {on?}` are
+message?, alt?, text?, background?: ""|matte|#rrggbb (#rrggbbaa and #rgba refused), hAlign?, vAlign?}` and `object.slice.clipToArtboard {on?}` are
 each one undo step. `view.slices.hide {hidden?}` and `view.slices.lock {locked?}` are session view state; the canvas
 draws the slices in the `sliceLineColor` preference, numbered while `showSliceNumbers` is on. Locking or hiding
 deselects the slices in every open document; unlocking or showing them again leaves them deselected and does not

docs/target-app-parity.md

@@ -1271,6 +1271,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
 - **Control transport:** idle sockets expire after 5 minutes, partial lines after an absolute 30 seconds from their first byte, and reply writes after 60 seconds (#1726). Real TCP regressions cover stalled peers and slot recovery; command execution has its separate existing timeout. MCP limits: 16 MiB lines, 64-message batches, 256-step `command_batch` with 8 MiB of kept results and 256 MiB control replies (#1764, fixes #1727). MCP requires the JSON-RPC 2.0 envelope and matches control replies by id, and an app refusal without an id reports the app's reason (#1768, fixes #1728). The MCP server reconnects to the app before a call when the connection was closed or holds stray bytes, after an app restart or an idle expiry (#1739).
 - **MCP vertical type:** `add_text {vertical: true}` forwards to `text.create` or `text.createInPath`. Regression coverage checks point type, rectangular area type, type in a path and type on a path with true, false and omitted direction. This exposes an existing engine option; it does not change glyph orientation, CJK composition or the type/automation scores.
 - **Malformed MCP replies:** invalid JSON from the app drops the connection, reports that the operation may have completed and leaves the next call to reconnect; it never replays the edit (#1761). TCP regressions cover malformed and uncorrelated replies, connection closure and a subsequent inspection. Envelope regressions also verify zero backend calls and unchanged initialization state for invalid versions.
+- **Command colors with an alpha part:** as VectorCraft's own rule, a command's color given as a hex color with an alpha part (`#rrggbbaa` or `#rgba`) is an error and changes nothing: fill and stroke colors, artboard, swatch, color group, new type, mesh point and Symbol Stainer colors, and the colors of `recolor.apply`, `select.color`, `color.gamutCheck`, `object.slice.options` (`background`), `object.rasterize` (`background`), `document.export` (`background`, `matte`) and `document.exportForWeb` (`matte`, `colorTable`) among them. Before, a `#rrggbbaa` color was set without its alpha, and a few commands used a default or skipped a color they could not read. The effects' `color` keeps taking `#rrggbbaa`. Tests: `a_color_with_an_alpha_part_is_refused` in `crates/engine/src/tests/panelcmds.rs`, `a_mesh_point_color_with_an_alpha_part_is_refused` and `a_stain_color_with_an_alpha_part_is_refused`.
 - **Tests and robustness:** ~2,840 automated tests: model-based property tests, a junk-parameter sweep over every command, import fuzzing (SVG, PDF, libraries), golden renders, and MCP end-to-end tests over stdio. Shipped code never panics (workspace lints and a rollback safety net; see `docs/development.md`). Scatter graph data near the largest number is refused with a message instead of hanging the app (#1780). One very long Liquify stroke segment can no longer create an unbounded number of dabs (#1779). A drag whose last preview failed is no longer recorded as a committed command in the journal that actions and scripts read (#1775). Tests: text import fuzzing no longer searches installed fonts (#1785). Tests: render tests that add fonts no longer race the frame cache tests (#1795).
 - **Raster-effect resolution in PDF:** when the 64 million pixel cap on an effect image renders an object's raster effects below the document's raster effects resolution, PDF export warns once per object with the requested and the lowest achieved ppi (`crates/engine/src/tests/pdfraster.rs`).
 - **Rasterize effect:** Effect › Rasterize… (`rasterize`) draws an object as an image of square pixels at its own resolution or the document's raster effects resolution, with Object › Rasterize's color model, background, anti-aliasing, clipping mask and room around the object, while its paths stay editable. Expand Appearance and PDF and SVG export of an object whose only raster effect is Rasterize give the image Object › Rasterize gives (`crates/engine/src/tests/rastersettings.rs`, `crates/render/src/tests/rasterize.rs`). Behavior reference: [How to save artwork in Illustrator](https://helpx.adobe.com/illustrator/using/saving-artwork.html), [Work with effects](https://helpx.adobe.com/illustrator/using/effects.html).