Add BAST document parser (typed tree, step 3)

- New src/bast.rs: typed surface over a BAST document — BastDoc,
  BastDef, BastDefKind, BastStruct, BastField, BastUnion,
  BastDiscriminator, BastEnum, BastType, BastRef, BastArray,
  BastRecord. Borrows from the source Value (no clone of the tree).
- $ref resolution is a single hash lookup against $defs
  (#/$defs/<name> only); union variant refs resolved lazily via
  BastDoc::resolve_typeref / resolve_ref — replaces
  normalize_refs + inline_union_variant_refs (those stay for now;
  step 8 removes them).
- Untrusted-input safe: every walk returns AlkTypeError::Schema on
  a malformed document, never panic/unwrap (AGENTS.md §3).
  Overflow-safe usize parsing via try_from (AGENTS.md §4).
- D-BAST-005 enforced: fields array only valid with field-name
  discriminators; required for them.
- Additive only: new pub mod bast + re-exports in lib.rs. No
  existing re-exports removed (those go in step 8). 47 new tests.

Verification:
- cargo test --release: 465 pass (379 lib + 86 integration)
- cargo clippy --all-targets -- -D warnings: clean
- cargo doc --no-deps: clean
- cargo build --target wasm32-unknown-unknown --release: clean

src/lib.rs

@@ -28,6 +28,7 @@
 
 #[macro_use]
 mod macros;
+pub mod bast;
 pub mod bast_meta;
 pub mod builder;
 pub mod data_access;
@@ -42,6 +43,10 @@ pub mod tunion;
 pub mod validation;
 
 pub use bast_meta::BAST_META_SCHEMA;
+pub use bast::{
+    BastArray, BastDef, BastDefKind, BastDiscriminator, BastDoc, BastEnum, BastField, BastRef,
+    BastRecord, BastStruct, BastType, BastUnion,
+};
 pub use builder::{Definitions, Discriminator, Schema};
 pub use engine::{LayoutMode, AlkTypeEngine};
 pub use error::AlkTypeError;