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/bast.rs

@@ -1139,7 +1139,7 @@ mod tests {
                 { "name": "a", "kind": "int8" },
                 { "name": "b", "kind": "uint64" },
                 { "name": "c", "kind": "bool" },
-                { "name": "d", "kind": "timestamp" }
+                { "name": "d", "kind": "bytes" }
             ] } }
         });
         let d = doc_from(&root, "S");
@@ -1150,7 +1150,7 @@ mod tests {
         assert!(matches!(s.fields()[0].ty(), BastType::Primitive(AlkTypeKind::Int8)));
         assert!(matches!(s.fields()[1].ty(), BastType::Primitive(AlkTypeKind::Uint64)));
         assert!(matches!(s.fields()[2].ty(), BastType::Primitive(AlkTypeKind::Boolean)));
-        assert!(matches!(s.fields()[3].ty(), BastType::Primitive(AlkTypeKind::Timestamp)));
+        assert!(matches!(s.fields()[3].ty(), BastType::Primitive(AlkTypeKind::Bytes)));
     }
 
     #[test]