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/data-access.md

@@ -100,7 +100,7 @@ impl SequentialReader {
 ```
 
 `read_field`/`write_field` on `AlkTypeEngine` work for the fixed-size
-primitive kinds and the length-prefixed `String`/`Bytes`/`Timestamp`
+primitive kinds and the length-prefixed `String`/`Bytes`
 fields. Composite kinds (`Struct`, `Union`, `Array`, `Record`) return a
 `FieldValue` carrying a layout descriptor (byte range, variant start,
 or array stride) for the consumer to recurse on — see §"FieldValue" above.