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).

src/builder.rs

@@ -170,10 +170,6 @@ impl Schema {
     pub fn bytes() -> Self {
         Self::primitive(AlkTypeKind::Bytes)
     }
-    /// BAST `timestamp` (length-prefixed RFC 3339 string).
-    pub fn timestamp() -> Self {
-        Self::primitive(AlkTypeKind::Timestamp)
-    }
 
     fn primitive(kind: AlkTypeKind) -> Self {
         Self {
@@ -837,7 +833,6 @@ mod tests {
         assert_eq!(Schema::boolean().build(), json!("bool"));
         assert_eq!(Schema::string().build(), json!("string"));
         assert_eq!(Schema::bytes().build(), json!("bytes"));
-        assert_eq!(Schema::timestamp().build(), json!("timestamp"));
     }
 
     // ----- Struct -------------------------------------------------------