Builder API produces BAST JSON (step 7)

- Schema internals: flat Map<String, Value> → Repr enum distinguishing
  BAST primitives (bare TypeRef strings), BAST composites (struct/
  union/enum/array/record objects), $ref, standard JSON Schema
  objects, and raw adopted values. Annotations (endian/align/encoding/
  maxLength) stored on the Schema and placed correctly by build()
  (struct-level) or field() (field-level).

- Primitive constructors (uint32, string, bytes, etc.) now produce
  bare BAST kind strings ("uint32") instead of {"AlkType:Uint32":true}.

- struct_().field(...) produces {"kind":"struct","fields":[{name,kind,...annos}]}
  with an ordered fields array (BAST design principle #4 — no reliance
  on preserve_order for field order).

- union_() emits {"kind":"union","discriminator":{...},"mapping":{...}}
  with BAST kind strings in the discriminator ("uint8" not
  "AlkType:Uint8"). Field-name discriminator unions emit the fields
  array; byte-offset unions omit it.

- enum_of() produces {"kind":"enum","values":[...]}.

- array_of(element).count(n) produces {"kind":"array","element":...,"count":n}.
  .count() is an additive method (D-BAST-004 requires count; the
  array_of signature is unchanged per the semver contract).

- record_of(values) produces {"kind":"record","values":...}.

- object()/string_()/etc. unchanged — standard JSON Schema output.

- encoding() now stores the value on the Schema; field() extracts it
  as a field-level property. No more keyword-object duality.

- max_length() on standard types emits the standard keyword; on BAST
  types it is extracted by field() as a field-level constraint.

- Definitions::build_doc(root_name, root) — new additive method
  producing a complete BAST document with the root type inside $defs
  (where BAST requires it). The old build()/merge_into() remain for
  backward compatibility.

- All builder tests updated to expect BAST output shapes. New tests:
  field annotations, field-name union with fields, array with count,
  ref element, encoding emission, Definitions::build_doc round-trip
  through compile(), SFTP-style union compile, array/record compile.

- Module doc comments updated (builder.rs, lib.rs).

Verification:
- cargo test --release: 427 pass (350 lib + 77 integration)
- cargo clippy --all-targets -- -D warnings: clean
- cargo doc --no-deps: clean (no warnings)
- cargo build --target wasm32-unknown-unknown --release: clean

src/lib.rs

@@ -26,9 +26,10 @@
 //!   `jsonschema::Validator` from a consumer-provided JSON Schema for
 //!   `validate_json` / `is_valid_json` (D-BAST-007) — no custom keywords,
 //!   no BAST involvement.
-//! - **Builder** ([`builder`]): Fluent Rust API for constructing alktype
-//!   JSON Schemas at runtime, producing `serde_json::Value` (ADR-009).
-//!   (Step 7 converts the builder to emit BAST JSON.)
+//! - **Builder** ([`builder`]): Fluent Rust API for constructing BAST
+//!   documents (binary layout) and standard JSON Schemas (JSON
+//!   validation) at runtime, producing `serde_json::Value` (ADR-009,
+//!   D-BAST-008). `struct_()` → BAST, `object()` → standard JSON Schema.
 //! - **Materialize** ([`materialize`]): Materialize a `serde_json::Value`
 //!   tree from a binary buffer by walking the BAST typed tree. Used by
 //!   `AlkTypeEngine::validate_bytes` (ADR-010).