Commands: the View commands check their params against machine-readable schemas

A new standalone crate, vectorcraft-apischema, describes a command's
parameters: type (number with range and unit, integer, boolean, string,
enum, id, id list, color, point, object, array), whether each is
required, the default of an omitted value, whether null counts as
omitted, the API levels it was added and deprecated at, and the lenient,
cost and writes fields. It validates a params object and names the
parameter of the first problem, writes the params text, writes the
schema in the shape of a JSON Schema 2020-12 object schema, and hashes a
set of schemas.

CommandSpec gains an optional schema (both builders: the cmd! macro and
the text style specs). Session::execute checks the params of a command
that has one before anything else, refusing a value of the wrong type,
out of range or not in its list, a missing parameter or an unknown key
with an error naming the parameter, and fills in defaults; the journal
keeps the params as given. engine.commands lists each schema.

The six commands in cmd/views.rs have schemas, and their params text is
the text the schema writes. schemas::API_VERSION is 1, and a test fails
when a schema changes without a new row in SCHEMA_HASHES.

Docs: docs/mcp.md (Parameter schemas), docs/target-app-parity.md.

docs/mcp.md

@@ -1253,6 +1253,34 @@ reference that names no value fails the step, so a command never runs with one l
 vectorcraft-cli run --cmd file.new --params '{"width":600,"height":200}'   --cmd text.create --params '{"x":10,"y":60,"text":"plain and bold words","size":30}'   --cmd text.setRangeStyle --params '{"id":"$1.id","start":10,"end":14,"style":"Bold"}'
 ```
 
+### Parameter schemas
+
+Some commands have a machine-readable parameter schema: at present `view.capsLock`, `view.saved.new`,
+`view.saved.edit`, `view.saved.list`, `view.transparencyGrid` and `view.snapToGlyph.options`. The engine's
+`engine.commands` list gives such a command a `schema` field: an object schema in the shape of JSON Schema 2020-12
+(https://json-schema.org/draft/2020-12), with `type`, `properties`, `required`, `additionalProperties: false`,
+`minimum` and `exclusiveMinimum` for ranges, `enum`, and `default` for a default the engine fills in. VectorCraft's own
+fields are `x-since` (the API level that added the command or the parameter), `x-deprecatedSince`, `x-lenient`,
+`x-unit`, `x-kind` (`id`, `color` or `point`), `x-default` (a default the command works out, such as `toggle`),
+`x-cost`, `x-writes` (`nothing`, `session`, `preferences` or `document`) and `x-returns`. A parameter that reads null
+as left out is an `anyOf` with `{"type": "null"}`. The command's `params` text is written from the same schema.
+
+Every caller, the desktop app and a step of `command.batch` included, has the params of such a command checked against
+its schema before the command runs. A value of the wrong type, out of range or not in its list, a missing required
+parameter or an unknown key is refused with a parameter error that names the parameter, with the path to a value
+inside a list or an object, and the document and its undo history stay as they were:
+
+```json
+{"id": "view.saved.new", "params": {"center": [0, 0], "zoom": 0}}
+→ invalid parameters for `view.saved.new`: `zoom` must be a number > 0, not 0
+{"id": "view.saved.edit", "params": {"edits": [{"name": "A", "delete": 1}]}}
+→ invalid parameters for `view.saved.edit`: `edits[0].delete` must be true or false, not 1
+```
+
+An omitted value with a `default` takes it (`rotation` 0 in `view.saved.new`); the journal keeps the params as they
+were given. The schemas belong to API level 1 (`vectorcraft_engine::schemas::API_VERSION`); a change to any schema
+raises the level.
+
 ## Checking scripts written for the reference app
 
 `script.check {path? | data? | dataBase64?, name?, includes?}` reads a script written for the reference app

docs/target-app-parity.md

@@ -617,6 +617,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
 - **Linux file associations:** the desktop entry lists the MIME type of every format File › Open reads but PSD, PSB, `.ps` PostScript, Targa, PCX, Pixar and JPEG 2000, `.ai`, `.ait` and EPS included, and the MIME package declares the types shared-mime-info lacks (VectorCraft documents and templates, `.ait`, Affinity), so file managers list VectorCraft under Open With for them, as Finder does on macOS. `the_linux_desktop_entry_opens_every_readable_format` in `apps/vectorcraft/src/tests.rs` checks both files against `OPEN_EXTS`. On Debian 13, Ubuntu 24.04 and Fedora 42, `desktop-file-validate` accepts the desktop entry, `update-mime-database` accepts the MIME package, and `gio` registers the app for every one of these types.
 
 - **Architecture:** 19+ crates with enforced layering (`cargo xtask layers`). Every action is a command (~400 engine + ~50 UI). Undo is unlimited via structural sharing. `command.batch` runs several commands as one transaction.
+- **Parameter schemas:** a command can carry a machine-readable parameter schema (the new `vectorcraft-apischema` crate): each parameter's type, range, unit, whether it is required, its default, whether null counts as left out, and the API levels it was added and deprecated at. `engine.commands` lists the schema in the shape of a JSON Schema 2020-12 object schema, the command's `params` text is written from it, and every caller has the params checked against it before the command runs, with an error naming the parameter and no change to the document. The View commands `view.capsLock`, `view.saved.new`, `view.saved.edit`, `view.saved.list`, `view.transparencyGrid` and `view.snapToGlyph.options` have schemas. The schemas belong to API level 1 (`vectorcraft_engine::schemas::API_VERSION`), and a test fails when a schema changes without a new level. Tests: `crates/apischema/src/tests.rs`, `schemas::tests` and `cmd::views::tests::each_view_command_refuses_a_bad_value_naming_the_parameter` in `vectorcraft-engine`.
 - **Automation:**
   - Variables: Update Data Set writes what the art holds into the active data set for the variables that data set holds, and keeps the data set's value for a variable whose objects are locked or inside a locked layer or group; `dataset.update {all: true}` also adds the other variables that the art reports a value for.
   - Variables: `dataset.select`, `dataset.next` and `dataset.prev` rewrite type and apply the other values on hidden objects and in hidden layers and groups, which stay hidden unless a visibility value shows them, so a layer shown later holds the selected data set's text; objects that are locked or in a locked layer or group are skipped.