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/engine.rs

@@ -984,10 +984,8 @@ mod tests {
     //
     // Note: step 4 wires the layout layer to BAST. The `validate_bytes`
     // value-constraint enforcement (maxLength, enum bounds) is step 5's
-    // concern (the BAST-native validator). Until step 5 lands, the
-    // validator built from a BAST doc sees no `AlkType:*` custom keywords,
-    // so jsonschema performs only structural validation. These tests
-    // cover the materialization + structural path.
+    // concern (the BAST-native validator). These tests cover the
+    // materialization + structural path.
 
     fn chunk_header_doc() -> Value {
         json!({