Remove the Timestamp kind

The Timestamp kind was a residual from an early research reference. It
was byte-identical to String everywhere (length-prefixed UTF-8) and its
only distinguishing behavior was a hand-rolled non-strict RFC 3339 check
that the docs admitted was incomplete (Feb 31 passes, seconds range
unchecked, no leap seconds). JSON-level timestamp validation is
jsonschema's job (format: date-time on the validate_json path), not
alktype's.

Removes the AlkTypeKind::Timestamp variant, its to_bast_str/from_bast_str
mapping, the builder's Schema::timestamp() constructor, the
validate_timestamp/is_rfc3339_timestamp validator arms, and the
materializer/reader/engine timestamp arms. Updates the meta-schema
primitive enum (14 -> 13), the spec docs (bast-format.md, schema-layer.md,
builder.md, validation.md, overview.md, data-access.md, README.md), and
the kind-count references (19 -> 18).

Verification: cargo test --release (407 pass), cargo clippy --all-targets
-- -D warnings (clean), cargo doc --no-deps (clean), cargo build --target
wasm32-unknown-unknown --release (clean).

docs/architecture/README.md

@@ -16,7 +16,7 @@ format definition; the engine is generic.
 |----------|--------|-------------|
 | [overview.md](overview.md) | accepted | Crate purpose, "schema is the format" principle, dependencies, consumers, scope boundaries |
 | [`bast-format.md`](bast-format.md) | accepted | **Normative BAST format specification.** Meta-schema, TypeRef, TypeDef shapes (Struct/Union/Enum/FieldDef), examples, validation model. The format the engine consumes. |
-| [schema-layer.md](schema-layer.md) | accepted | The BAST parser (`src/bast.rs`) — the typed tree (`BastDoc`/`BastDef`/`BastType`/…) every engine module walks, the 19 BAST kinds, the `AlkTypeKind` enum, and the foundational annotation types. |
+| [schema-layer.md](schema-layer.md) | accepted | The BAST parser (`src/bast.rs`) — the typed tree (`BastDoc`/`BastDef`/`BastType`/…) every engine module walks, the 18 BAST kinds, the `AlkTypeKind` enum, and the foundational annotation types. |
 | [layout-engine.md](layout-engine.md) | draft | Offset computation, the two layout modes (packed sequential vs aligned static), alignment, endianness, variable-length handling |
 | [data-access.md](data-access.md) | draft | Read/write functions, TUnion dispatch, field paths, zero-copy access, length-prefix reading |
 | [validation.md](validation.md) | accepted | The two-validator model (BAST-native for `validate_bytes`, standard `jsonschema` for `validate_json`), `AlkTypeError`, load-time vs access-time validation, `AlkTypeEngine` as the compiled form of a BAST document (ADR-010, ADR-VAL-SPLIT). |