Embed BAST v1 meta-schema as BAST_META_SCHEMA

Step 2 of the BAST pivot. Adds the normative BAST meta-schema
(Draft 2020-12 JSON Schema) as a public serde_json::Value, embedded
at compile time and available for validating BAST document
well-formedness. Copied verbatim from docs/architecture/bast-format.md
§The Meta-Schema.

- New src/bast_meta.rs: BAST_META_SCHEMA static, built lazily via
  LazyLock (the json! macro allocates, so it can't be a const;
  parsed once, reused as &'static Value thereafter).
- src/lib.rs: pub mod bast_meta + re-export BAST_META_SCHEMA.
  Additive public surface.

The meta-schema validates structure (correct $defs shape, known
kind strings, required properties, no additional properties,
$ref restricted to #/$defs/<name>). Value-domain constraints
(maxLength, enum index bounds) are enforced by the BAST-native
validator (step 5), not this meta-schema.

Verification:
- cargo test --release: 418 tests pass (332 lib + 86 integration);
  16 new unit tests exercise the meta-schema against valid and
  invalid BAST documents (missing $defs, unknown kind, additional
  properties, byte-discriminator union, enum, array+count, record,
  $ref, malformed ref, empty enum).
- cargo clippy --all-targets -- -D warnings: clean.
- cargo build --target wasm32-unknown-unknown --release: clean
  (LazyLock + serde_json::json! macro are wasm-safe).

src/lib.rs

@@ -28,6 +28,7 @@
 
 #[macro_use]
 mod macros;
+pub mod bast_meta;
 pub mod builder;
 pub mod data_access;
 pub mod engine;
@@ -40,6 +41,7 @@ pub mod sequential_reader;
 pub mod tunion;
 pub mod validation;
 
+pub use bast_meta::BAST_META_SCHEMA;
 pub use builder::{Definitions, Discriminator, Schema};
 pub use engine::{LayoutMode, AlkTypeEngine};
 pub use error::AlkTypeError;