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)));

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 {

src/ir/interp.test.zig

@@ -713,3 +713,50 @@ test "comptime: type_tag comparison" {
     const r_false = try interp.call(FuncId.fromIndex(1), &.{});
     try std.testing.expectEqual(false, r_false.asBool().?);
 }
+
+// ── Test: type_name builtin reads .type_tag, returns the typeName ───────
+
+test "comptime: type_name builtin on type_tag" {
+    const alloc = std.testing.allocator;
+    var module = Module.init(alloc);
+    defer module.deinit();
+    var b = Builder.init(&module);
+
+    _ = b.beginFunction(str(&module, "test_type_name"), &.{}, .string);
+    const entry = b.appendBlock(str(&module, "entry"), &.{});
+    b.switchToBlock(entry);
+    const t = b.constType(.s64);
+    var args = [_]inst_mod.Ref{t};
+    const r = b.callBuiltin(.type_name, &args, .string);
+    b.ret(r, .string);
+    b.finalize();
+
+    var interp = Interpreter.init(&module, alloc);
+    defer interp.deinit();
+    const result = try interp.call(FuncId.fromIndex(0), &.{});
+    try std.testing.expectEqualStrings("s64", result.asString(&interp).?);
+}
+
+// ── Test: type_eq builtin on two .type_tag operands ────────────────────
+
+test "comptime: type_eq builtin on type_tag values" {
+    const alloc = std.testing.allocator;
+    var module = Module.init(alloc);
+    defer module.deinit();
+    var b = Builder.init(&module);
+
+    _ = b.beginFunction(str(&module, "test_type_eq_builtin"), &.{}, .bool);
+    const entry = b.appendBlock(str(&module, "entry"), &.{});
+    b.switchToBlock(entry);
+    const a = b.constType(.string);
+    const c = b.constType(.string);
+    var args = [_]inst_mod.Ref{ a, c };
+    const r = b.callBuiltin(.type_eq, &args, .bool);
+    b.ret(r, .bool);
+    b.finalize();
+
+    var interp = Interpreter.init(&module, alloc);
+    defer interp.deinit();
+    const result = try interp.call(FuncId.fromIndex(0), &.{});
+    try std.testing.expectEqual(true, result.asBool().?);
+}

src/ir/interp.zig

@@ -1740,6 +1740,43 @@ pub const Interpreter = struct {
             .type_of => return bailDetail("comptime #builtin type_of: handled at lowering, not the interp"),
             .alloc => return bailDetail("comptime #builtin alloc unused (use context.allocator.alloc)"),
             .dealloc => return bailDetail("comptime #builtin dealloc unused (use context.allocator.dealloc)"),
+
+            // ── Comptime reflection (Type-as-Value path) ─────────
+            // These are only reached when lower.zig emitted a real
+            // builtin_call — i.e. the type argument was NOT statically
+            // resolvable (e.g. inside a builder body where `args[i]` is
+            // a `.type_tag(TypeId)` Value bound at interp time). Static
+            // calls fold to `const_string` / `const_bool` at lower time
+            // and never hit this dispatch.
+            .type_name => {
+                if (bi.args.len < 1) return bailDetail("comptime type_name: missing argument");
+                const arg = frame.getRef(bi.args[0]);
+                const tid = arg.asTypeId() orelse return bailDetail("comptime type_name: argument is not a Type value (expected `.type_tag`, got a different Value kind)");
+                const name = self.module.types.typeName(tid);
+                // Copy the slice into the interp's allocator so it
+                // outlives any TypeTable churn during the rest of the
+                // interp execution. The TypeTable's strings are stable
+                // for now but copying is the safe pattern.
+                const owned = self.alloc.dupe(u8, name) catch return error.CannotEvalComptime;
+                return .{ .value = .{ .string = owned } };
+            },
+            .type_eq => {
+                if (bi.args.len < 2) return bailDetail("comptime type_eq: needs two Type arguments");
+                const a = frame.getRef(bi.args[0]).asTypeId() orelse return bailDetail("comptime type_eq: first argument is not a Type value");
+                const b = frame.getRef(bi.args[1]).asTypeId() orelse return bailDetail("comptime type_eq: second argument is not a Type value");
+                return .{ .value = .{ .boolean = a == b } };
+            },
+            .has_impl => {
+                // has_impl at interp time needs access to the host's
+                // protocol-registration maps (protocol_thunk_map +
+                // param_impl_map). These live on `Lowering`, not on
+                // the Interpreter. Plumbing a queryable snapshot is
+                // its own slice — until then, bail loudly so the user
+                // gets a clear "not yet wired" message instead of a
+                // silent false. Static-arg has_impl still works via
+                // `tryConstBoolCondition` in lower.zig.
+                return bailDetail("comptime has_impl: interp-time evaluation not yet wired (use static type args for now — they fold at lower time)");
+            },
         }
     }