ffi M5.A.next.4.1: interp arms for reflection builtins on .type_tag

Second slice of the .type_tag activation. The reflection
intrinsics (`type_name`, `type_eq`, `has_impl`) now have
interp-time implementations that read `.type_tag` Values
directly. Today's lower-time fast path (folding to
`const_string`/`const_bool` when the type arg is statically
resolvable) stays — these interp arms are the fallback path
for when lowering emits a real `builtin_call` because the
arg is interp-time-only (e.g. `args[i]` inside a builder body
where the pack element is bound at interp execution).

Plumbing:
- New BuiltinId entries: `type_name`, `type_eq`, `has_impl`.
- Interp arms in `execBuiltinInner`:
  - `type_name(t)`: reads `.type_tag` via `asTypeId`, looks up
    via `module.types.typeName`, dupes the slice into the
    interp allocator, returns `.string`. Non-`.type_tag` arg
    → `bailDetail` ("argument is not a Type value").
  - `type_eq(a, b)`: both args must be `.type_tag`; compares
    TypeIds. Either side missing → `bailDetail`.
  - `has_impl(P, T)`: bails with a "not yet wired" message —
    interp-time has_impl needs a queryable snapshot of the
    host's `protocol_thunk_map` + `param_impl_map`, which is
    its own follow-up slice. Static-arg has_impl still works
    via the lower-time `tryConstBoolCondition` fast path.
- emit_llvm: explicit arms for the three new builtins that
  log + map to undef-i64 (Type values are comptime-only; if
  one of these reaches LLVM emit, lowering produced wrong
  IR — the LLVM verifier downstream surfaces the offending
  site).

Three new Zig unit tests in interp.test.zig:
- `type_name builtin on type_tag` — emits a `builtin_call`
  to `type_name` with a `const_type(s64)` operand, asserts
  the result is the string "s64".
- `type_eq builtin on type_tag values` — two equal Type
  operands compare equal.
- (Pre-existing) `const_type yields type_tag` + `type_tag
  comparison` from 4.0 still pass.

208/208 example tests + `zig build test` green. No source-
language path constructs `.type_tag` yet — the foundation is
ready for the `$args`-in-expression-position slice that
turns it on for users.

src/ir/inst.zig

@@ -367,6 +367,17 @@ pub const BuiltinId = enum(u16) {
     type_of,
     alloc,
     dealloc,
+    // Comptime-only reflection builtins. Today's `tryLowerReflectionCall`
+    // folds these at lower time when the type argument is statically
+    // resolvable — emits a `const_string` / `const_bool` directly.
+    // These BuiltinId entries are the FALLBACK path: when the arg is
+    // a runtime/interp-time value (e.g. `args[i]` inside a builder
+    // body, carrying a `.type_tag(TypeId)` only at interp execution),
+    // lowering emits a `builtin_call` to one of these. The interp
+    // implements them; emit_llvm bails (Type is comptime-only).
+    type_name,
+    type_eq,
+    has_impl,
 };
 
 pub const CompilerCall = struct {