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/overview.md

@@ -27,8 +27,8 @@ three capabilities:
 3. **Validation** — two validators for two input types:
    - `validate_bytes(&[u8])` uses the BAST-native validator (a recursive
      walker over the BAST type tree) to check the value-domain
-     constraints the materializer doesn't (integer ranges, `maxLength`,
-     timestamp shape, enum index bounds, union variant constraints).
+      constraints the materializer doesn't (integer ranges, `maxLength`,
+      enum index bounds, union variant constraints).
    - `validate_json(&Value)` uses a standard `jsonschema::Validator`
      compiled from a consumer-provided JSON Schema (BAST is not involved
      — BAST describes bytes, not JSON shape).
@@ -194,7 +194,7 @@ and [ADR-BAST](decisions/bast-bast-format.md).
 ## Architecture (component pointers)
 
 - **[schema-layer.md](schema-layer.md)** — the BAST parser (the typed
-  surface every engine module walks), the 19 BAST kinds, the
+  surface every engine module walks), the 18 BAST kinds, the
   `AlkTypeKind` enum, and the foundational annotation types.
 - **[`bast-format.md`](bast-format.md)** — the normative BAST format
   specification (meta-schema, TypeRef, examples, validation model).