feat(types): nominal identity + key-safe TypeTable mutation, ban raw update [stdlib D]

Phase D of the unified resolver: make the TypeTable safe to key by nominal
identity before same-name type shadows land (Phase E). Behavior-preserving —
nominal_id=0 means structural (today's keying, byte-identical); single-author
names intern to the same TypeId as before.

types.zig:
- StructInfo/EnumInfo/UnionInfo/TaggedUnionInfo/ErrorSetInfo gain
  `nominal_id: u32 = 0`. hash/eql fold it into the nominal arms ONLY, and only
  when nonzero, so legacy (structural) interning hashes/compares byte-identically.
- internNominal(info, nominal_id): stamps the id into the nominal arm then
  interns; nonzero id on a non-nominal info trips an assert.
- updatePreservingKey(id, info): field-fill that asserts the intern key is
  unchanged (replaces the forward-decl stub→full pattern).
- replaceKeyedInfo(id, info): the one legitimate re-key (anon rename
  __anon → Parent.field); removes the stale key and installs the new one.
- findUniqueByName: quarantined findByName that asserts ≤1 match.
- type_decl_tids: decl-node → TypeId identity map (the fn_decl_fids analogue),
  consumed by the resolver in Phase E.

Ban raw TypeTable.update outside types.zig (the acceptance bar): every caller
in lower.zig / type_bridge.zig / protocols.zig is reclassified — forward-decl
field fills route through updatePreservingKey, qualifyAnonType's rename through
replaceKeyedInfo. The raw `update` method is removed. Inline named type-decl
registration ("current winners") routes through internNominal(info, 0).

Tests (types.test.zig): forward-decl field fill (stable key), anon rename
(re-key), generic struct instantiation, type-returning function, parameterized
protocol value struct, same display-name → distinct nominal ids, plus an
old==new assertion (internNominal(.,0) byte-identical to legacy intern),
findUniqueByName, and the type_decl_tids identity map.

Gate: zig build (0), zig build test (421/421), run_examples (477, byte-identical),
m3te ios-sim build via worktree binary (0). No shadows registered; stubs intact.

src/ir/protocols.zig

@@ -282,7 +282,7 @@ pub const ProtocolResolver = struct {
 
         const struct_info: types.TypeInfo = .{ .@"struct" = .{ .name = name_id, .fields = fields.items, .is_protocol = true } };
         const id = if (table.findByName(name_id)) |existing| existing else table.intern(struct_info);
-        table.update(id, struct_info);
+        table.updatePreservingKey(id, struct_info);
 
         // Build protocol method info for dispatch
         var method_infos = std.ArrayList(ProtocolMethodInfo).empty;