Remove v0.1.0 custom-keyword machinery (step 8)

The BAST parser (step 3) and BAST-native validator (step 5) replaced
the v0.1.0 custom-keyword accessor layer; step 7 moved the builder to
BAST output. This step removes the now-dead code:

Removed from src/schema.rs:
- get_alktype_kind / get_alktype_kind_enum /
  get_alktype_kind_loose / get_alktype_kind_loose_enum
  (replaced by the BAST parser's kind dispatch)
- normalize_refs / inline_union_variant_refs + helpers
  (BAST refs are always #/$defs/<name>; resolution is a single
  hash lookup, variant refs resolve lazily)
- parse_encoding / parse_align / parse_max_length / parse_endian
  (bast.rs has its own BAST-property-form copies)
- parse_discriminator + DiscriminatorKind
  (replaced by bast::BastDiscriminator; builder has its own
  Discriminator enum)
- resolve_ref / resolve_ref_or_inline
  (replaced by BastDoc::lookup_def / resolve_typeref)
- FromStr impl, as_str, Endian::from_schema, ALKTYPE_PREFIX,
  BYTE_DISCRIMINATOR_TYPES, and the associated unit tests

Kept: AlkTypeKind enum + methods (type_size, natural_alignment,
is_fixed_size, needs_endian, is_composite, is_variable_length,
to_bast_str, from_bast_str), Display (now backed by to_bast_str),
Endian, VariableEncoding, U32_SIZE, DISCRIMINATOR_PATH.

src/lib.rs: dropped the 13 schema::* helper re-exports and
DiscriminatorKind from the public surface; kept Endian, AlkTypeKind,
VariableEncoding.

Doc/comment updates: bast.rs, builder.rs, engine.rs, error.rs —
removed references to the deleted functions and the AlkType:*
keyword form.

The jsonschema crate remains a dependency (validate_json path +
BAST meta-schema validation); build_validator was already repurposed
in step 6 (no custom keywords).

Verification:
- cargo test --release: 389 pass (312 lib + 77 integration)
- cargo clippy --all-targets -- -D warnings: clean
- cargo doc --no-deps: clean
- cargo build --target wasm32-unknown-unknown --release: clean

src/bast.rs

@@ -1,7 +1,6 @@
 //! BAST document parser — the typed surface over a BAST (Binary Abstract
 //! Syntax Tree) document that the layout engines, materializer, and
-//! BAST-native validator walk instead of the v0.1.0 `get_alktype_kind*`
-//! custom-keyword accessors.
+//! BAST-native validator walk.
 //!
 //! This is step 3 of the BAST pivot
 //! ([`docs/plans/bast-implementation.md`](../docs/plans/bast-implementation.md)).
@@ -19,11 +18,10 @@
 //! parse allocation-free beyond the small typed nodes themselves.
 //!
 //! `$ref` resolution is a single hash lookup against `$defs` — BAST refs
-//! are always full JSON Pointers restricted to `#/$defs/<name>` (no
-//! `normalize_refs`, no `inline_union_variant_refs`; both are removed in
-//! step 8). Union variant `$ref`s are resolved **lazily** by the
-//! materializer/validator via [`BastDoc::resolve_typeref`]; the parser
-//! only records the [`BastRef`] target name.
+//! are always full JSON Pointers restricted to `#/$defs/<name>`. Union
+//! variant `$ref`s are resolved **lazily** by the materializer/validator
+//! via [`BastDoc::resolve_typeref`]; the parser only records the
+//! [`BastRef`] target name.
 //!
 //! ## Untrusted input
 //!
@@ -91,8 +89,8 @@ impl<'a> BastDoc<'a> {
 
     /// Look up a raw `$defs` entry by name. Returns the raw JSON node.
     ///
-    /// The single hash lookup that replaces v0.1.0's `normalize_refs` +
-    /// `resolve_ref` machinery. BAST refs are always `#/$defs/<name>`;
+    /// The single hash lookup that replaces v0.1.0's ref-normalization
+    /// machinery. BAST refs are always `#/$defs/<name>`;
     /// no external refs, no fragment-only pointers, no bare names.
     pub fn lookup_def(&self, name: &str) -> Result<&'a Value, AlkTypeError> {
         Self::lookup_def_raw(self.root, name)
@@ -102,8 +100,8 @@ impl<'a> BastDoc<'a> {
     ///
     /// Used lazily by the materializer and BAST-native validator (step 5)
     /// for union variant dispatch and `$ref` fields. The parser does not
-    /// inline refs at construction time — this is the replacement for
-    /// v0.1.0's `inline_union_variant_refs`.
+    /// inline refs at construction time — variant refs stay as
+    /// [`BastRef`]s and resolve on demand.
     pub fn resolve_ref(&self, r: &BastRef<'a>) -> Result<BastDef<'a>, AlkTypeError> {
         let raw = self.lookup_def(r.name())?;
         BastDef::parse(raw, r.name(), "")
@@ -440,8 +438,7 @@ impl<'a> BastField<'a> {
 /// discriminator.
 ///
 /// Variant `$ref`s are resolved **lazily** by the materializer/validator
-/// via [`BastDoc::resolve_typeref`] — no compile-time inlining (the
-/// replacement for v0.1.0's `inline_union_variant_refs`).
+/// via [`BastDoc::resolve_typeref`] — no compile-time inlining.
 #[derive(Debug, Clone)]
 pub struct BastUnion<'a> {
     endian: Endian,
@@ -774,8 +771,8 @@ impl<'a> BastType<'a> {
 /// A `$ref` to a named `$defs` entry, restricted to `#/$defs/<name>`.
 ///
 /// The restriction keeps resolution a single hash lookup
-/// ([`BastDoc::lookup_def`]) and eliminates the `normalize_refs` step
-/// the v0.1.0 engine needed for TypeBox's bare-name refs.
+/// ([`BastDoc::lookup_def`]); the v0.1.0 engine needed a normalization
+/// pass for TypeBox's bare-name refs, but BAST forbids them.
 #[derive(Debug, Clone)]
 pub struct BastRef<'a> {
     name: &'a str,
@@ -892,11 +889,11 @@ impl<'a> BastRecord<'a> {
 // Shared annotation parsers (BAST-property form).
 //
 // These read type-level/field-level properties (`endian`, `align`,
-// `encoding`, `maxLength`) directly off a BAST node, instead of the
-// v0.1.0 keyword-value objects that `schema::parse_*` read. Their
-// *semantics* are unchanged (ADR-003); only the input location moved.
-// They're `fn`s rather than methods on the typed nodes so the layout
-// engines can call them on raw `&Value` during the step 4 migration.
+// `encoding`, `maxLength`) directly off a BAST node. Their *semantics*
+// are unchanged (ADR-003); only the input location moved from the
+// v0.1.0 keyword-value objects to BAST type-level properties. They're
+// `fn`s rather than methods on the typed nodes so the layout engines
+// can call them on raw `&Value` during the step 4 migration.
 // ---------------------------------------------------------------------------
 
 fn parse_endian_opt(node: &Value) -> Option<Endian> {