Share the schema vocabulary between the hosts

[zoosky]

Step 4 of specs/features/cli-and-host-seams.md: crates/accent-proust-schema-config, the declarative half of a schema written once, and the WebAssembly host moved onto it in the same PR — extracting without moving the only existing consumer would have left two copies, which is the state this step exists to prevent. Conformance holds at 95 green, 10 annotated, 0 failing (of 105).

What the crate is

The key lists (TOP_LEVEL, SCHEMA_KEYS, ATTRIBUTE_KEYS, SLOT_KEYS), the refusal of an unknown key with the path to it, and the mapping from a declaration onto Schema, SchemaAttribute, SchemaSlot, ValidationType, RenderPolicy, ErrorLevel and SchemaMatches. declare(&root) -> Result<Declared, Error>; Declared::apply(&mut MapSchemaSource) -> Option<Variables> does the one thing both hosts do identically — merge over the built-ins, a redeclared key keeping its position and taking the new value, which is what the corpus's fence-hook case depends on.

The one design decision (decision 10 in the spec)

The crate walks a Declaration trait, not a Value. Converting a host's whole configuration into the library's Value first would fail on a JavaScript function written under validate at the conversion, with a message about functions — and the wasm tests pin a message about hooks, at the key. Seven methods (shape, as_bool, as_str, keys, get, items, to_value) let the walk refuse keys before it reads any value, and convert a value only where the vocabulary carries one through (default, variables). Each host keeps its own reading, which is what the spec's "each keeps only its own deserialisation" meant. Value implements the trait too, so the crate's own tests need no host.

Errors

Error { path, kind } with a #[non_exhaustive] ErrorKind. The crate's Display says what is true everywhere; a host matches the kind to add the reason that is its own. The wasm host does exactly that for transform/validate, functions and partials, and every path and message its tests assert survives unchanged — config.tags.callout.validate + "code does not cross", config.tags.callout.attributes.type.type + "Str", the node-name list, "looked up by its name", "regular expression", config.variables.when.

The wasm host

config.rs goes from 438 lines to 166: a newtype Js(JsValue) implementing the trait, build in six lines, and explain. path.rs is gone (the shared Path has the same scheme); value.rs returns the shared Error and drops its own variables(). Unions are one level deep now — a list inside a list is refused — where the old walker recursed; upstream never nests, and the walk is otherwise as deep as the document chose.

Tests

16 in crates/accent-proust-schema-config/tests/declare.rs, over Value: every key mapped, each level's unknown-key path and expected list, nodes.tag refused, children: ["tag"] allowed, unknown node type with the list, unknown attribute type with its path, one-level unions, matches as a pattern, render: true on a schema, wrong shapes, variables carried whole, NoCounterpart's message, apply keeping position and taking the value, and the key lists as the format. None panics. The wasm suite (11 + 17) passes unchanged.

Workspace

Member, not default member, with its own Schema config CI job, gate commands and rows in AGENT.md. Depends on the library by path alone, like the other members.

Review fixes

Eight findings from /code-review, all taken; three were regressions against the wasm host's previous behaviour and each is pinned by a new wasm test:

• A throwing getter is refused, not dropped. Declaration::get now returns Result<Option<Self>, Error>, with an Unreadable kind; the host maps a failed Reflect::get to it. Absent and unreadable are different answers, and the second used to read as the first.
• An undefined variable is null again, as the deleted variables() had it.
• Any object is a schema when read key by key — a class instance, Object.create(base), a proxy. The plain-object strictness belongs to the value converter, which still applies it to default and variables.
• Expected { what, got }: the message names the shape found ("expected an object, not a string"), and the host matches Shape::Other("function") to say code does not cross — which restores the two reasons the extraction had lost (a custom attribute type, a pattern in matches).
• Error::explained(reason) is the seam; explain no longer retypes the vocabulary's sentence.
• A non-string errorLevel is a wrong shape, not the unknown name "".
• CHANGELOG names the one narrowed input (a nested type list); AGENT.md and the root manifest count the members rather than three.