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/emit_llvm.zig

@@ -2963,6 +2963,18 @@ pub const LLVMEmitter = struct {
                         _ = c.LLVMBuildCall2(self.builder, self.getWriteType(), write_fn, &write_args, 3, "");
                         self.advanceRefCounter();
                     },
+                    .type_name, .type_eq, .has_impl => {
+                        // Comptime-only reflection builtins. Reaching
+                        // LLVM emit means lowering DIDN'T fold the call
+                        // (static-arg fast path) AND lowering emitted a
+                        // real `builtin_call` — but the resulting IR
+                        // shouldn't survive past the comptime interp
+                        // that's supposed to consume it. Loud failure:
+                        // log + undef placeholder so the LLVM verifier
+                        // catches downstream use.
+                        std.debug.print("emit_llvm: comptime reflection builtin '{s}' reached runtime emit — Type values are interp-only.\n", .{@tagName(bi.builtin)});
+                        self.mapRef(c.LLVMGetUndef(self.toLLVMType(instruction.ty)));
+                    },
                     else => {
                         // size_of, cast — handled by lowering or codegen glue
                         self.mapRef(c.LLVMGetUndef(self.toLLVMType(instruction.ty)));