Validate BAST documents against the meta-schema at compile time

Resolves review #003 finding M3.

- Add bast_meta::validate_bast_doc and call it from AlkTypeEngine::compile
  before parsing. Malformed annotations (unknown endian/encoding strings,
  non-integer align/maxLength, missing required properties) now surface as
  AlkTypeError::Schema instead of being silently tolerated by the parser.
- Align the meta-schema TypeRef with the parser: allow inline struct/union/
  enum as TypeRefs (the parser and builder already accepted them; the
  meta-schema and spec did not). Update bast-format.md TypeRef table and
  the BastType doc comment to the seven-form vocabulary.

Verification: cargo test --release (410 pass), cargo clippy --all-targets
-- -D warnings (clean), cargo doc --no-deps (clean), cargo build --target
wasm32-unknown-unknown --release (clean).

src/bast.rs

@@ -681,12 +681,12 @@ impl<'a> BastEnum<'a> {
 /// A type reference — the central mechanism for typing fields, array
 /// elements, record values, and union variants.
 ///
-/// Four forms, matching the
+/// Seven forms, matching the
 /// [TypeRef](../docs/architecture/bast-format.md#typeref) `oneOf`:
-/// primitive string, `$ref` object, array object, record object.
-/// Composite kinds (`struct`/`union`/`enum`) only appear inline here when
-/// a field/element/value is an anonymous composite; named composites go
-/// through [`BastType::Ref`].
+/// primitive string, `$ref` object, array object, record object, and
+/// inline `struct`/`union`/`enum`. Composite kinds (`struct`/`union`/
+/// `enum`) appear inline here when a field/element/value is an anonymous
+/// composite; named composites go through [`BastType::Ref`].
 #[derive(Debug, Clone)]
 pub enum BastType<'a> {
     /// A built-in primitive (`"uint32"`, `"string"`, etc.).