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/validation.md

@@ -48,7 +48,6 @@ tree ([`crate::bast::BastDoc`]/[`BastType`]) that checks exactly these:
 | Float finiteness (Float32/64) | `validate_float` with `as_f64().is_finite()` |
 | String `maxLength` (byte length) | `check_string` reads the field-level `maxLength` |
 | Bytes `maxLength` (array length) | `check_bytes` accepts the `Value::Array` form (the materializer emits bytes as an array of u8) |
-| RFC 3339 timestamp shape | `validate_timestamp` — non-strict check (matching v0.1.0) |
 | Enum index bounds | `validate_enum` checks `idx < values.len()` — **fixes the v0.1.0 dead constraint** |
 | Union variant dispatch | `validate_union` reads `__discriminator`, looks up the variant, recurses via `validate_typeref` |
 | Struct fields | `validate_struct` walks `fields`, requires each declared field present, recurses |