ffi M5.A.next.4.0: activate Value.type_tag — opcode + helper + cmp

Wires the dormant `Value.type_tag(TypeId)` variant in interp.zig
so Type values flow through the comptime interpreter as
first-class kind-distinguished entities. No source-language
construction path yet — that's a follow-up. This commit is the
infrastructure foundation.

Audit findings (from interp.zig switch-walk):
- Every `else =>` arm over Value is either already loud
  (`bailDetail` / `error.TypeError`) or a pass-through helper
  (`materializeCtxArg`, `materializeForCall`, `resolveSlotChain`)
  where transit-unchanged is semantically correct for type_tag.
  No new silent paths introduced by activating the variant.
- The three pre-existing `.type_tag => return bailDetail(...)`
  arms (store-at-raw-ptr, deref-non-pointer, unbox-non-aggregate)
  already cover the disallowed paths cleanly.

New plumbing:
- `Op.const_type: TypeId` — dedicated opcode. Never piggybacks
  on `const_int`. Result IR-type is `.any` to signal "untyped
  at runtime" so downstream coercions fail loudly.
- `Builder.constType(tid)` constructor.
- Interp arm emits `Value{ .type_tag = tid }` for the op.
- emit_llvm arm bails loudly + emits an undef-i64 placeholder
  (Type is comptime-only — if a Type ever reached LLVM emit,
  some upstream builder leaked through; the diagnostic + LLVM
  verifier downstream surface the offending site).
- `print.zig` arm prints `const type(<typeName>)`.
- `Value.asTypeId() ?TypeId` helper — the kind-honest accessor
  for Type values. asInt/asFloat/asBool/asString continue to
  return null for `.type_tag` (no silent coercion).
- `evalCmp` arm for `.type_tag, .type_tag` — TypeId equality.
  Mixed `.type_tag` vs `.int` deliberately falls through to
  the typeErrorDetail bail (a Type is not an int).

Tests (src/ir/interp.test.zig):
- `const_type yields type_tag` — confirms the variant is
  produced and that asTypeId/asInt distinguish correctly.
- `type_tag comparison` — exercises cmp_eq on equal and
  unequal pairs, asserts the right bool comes back.

208/208 example tests + `zig build test` green. No user-visible
behaviour change yet — `.type_tag` is constructible from Zig-
side IR builders but no sx-level syntax produces it. Next slice
wires `$args` lowering (or `$args[i]` in expression position)
to emit `const_type` per pack element.

src/ir/module.zig

@@ -378,6 +378,16 @@ pub const Builder = struct {
         return self.emit(.const_undef, ty);
     }
 
+    /// Comptime-only Type value. Produces a `Value.type_tag(tid)` in
+    /// the interp; bails loudly in LLVM emit (Type is comptime-only).
+    /// The result-Ref's IR type is `.any` to flag the value as
+    /// "untyped at runtime" — emitters that try to coerce it will
+    /// fail loudly rather than silently materialise the TypeId as an
+    /// int.
+    pub fn constType(self: *Builder, tid: TypeId) Ref {
+        return self.emit(.{ .const_type = tid }, .any);
+    }
+
     // ── Arithmetic ──────────────────────────────────────────────────
 
     pub fn add(self: *Builder, lhs: Ref, rhs: Ref, ty: TypeId) Ref {