fix: diagnose unknown generic #builtin instead of silently returning 0 (issue 0144)

A bodiless #builtin with a $T: Type param routes through monomorphization.
When resolveBuiltin returned null for an unrecognized name, the builtin-body
branch fell through to ensureTerminator's constInt(0) -- a silent-fallback
default the CLAUDE.md REJECTED PATTERNS forbid. Emit a loud
'error: unknown #builtin <name>' diagnostic instead.

Regression: examples/1189-diagnostics-unknown-builtin.sx

examples/1189-diagnostics-unknown-builtin.sx

@@ -0,0 +1,16 @@
+// A bodiless `#builtin` carrying a `$T: Type` parameter, whose name the
+// compiler does not recognize, must fail LOUDLY with an "unknown #builtin"
+// diagnostic — not silently evaluate to 0 (the CLAUDE.md silent-fallback
+// pattern). The generic monomorphization path (monomorphizeFunction's
+// builtin-body branch) now diagnoses an unresolved builtin name instead of
+// falling through to `ensureTerminator`'s `constInt(0)`.
+//
+// Regression (issue 0144).
+#import "modules/std.sx";
+
+// `mystery` is not a recognized builtin.
+mystery :: ($T: Type, x: T) -> T #builtin;
+
+main :: () {
+    print("mystery(42) = {}\n", mystery(i64, 42));
+}

examples/expected/1189-diagnostics-unknown-builtin.exit

@@ -0,0 +1 @@
+1

examples/expected/1189-diagnostics-unknown-builtin.stderr

@@ -0,0 +1,5 @@
+error: unknown #builtin 'mystery'
+  --> examples/1189-diagnostics-unknown-builtin.sx:12:1
+   |
+12 | mystery :: ($T: Type, x: T) -> T #builtin;
+   | ^^^^^^^

examples/expected/1189-diagnostics-unknown-builtin.stdout

@@ -0,0 +1 @@
+

issues/0144-unrecognized-type-param-builtin-silent-zero.md

@@ -1,5 +1,13 @@
 # 0144 — unrecognized `$T`-param `#builtin` silently returns 0
 
+> **RESOLVED.** The generic monomorphization path (`monomorphizeFunction`'s
+> `builtin_expr` body branch in `src/ir/lower/generic.zig`) no longer falls
+> through to `ensureTerminator`'s silent `constInt(0)` when `resolveBuiltin`
+> returns null for an unknown name. It now emits a loud
+> `error: unknown #builtin '<name>'` diagnostic — removing exactly the
+> silent-fallback-default the CLAUDE.md REJECTED PATTERNS forbid. Regression
+> test: `examples/1189-diagnostics-unknown-builtin.sx`.
+
 ## Symptom
 
 A **bodiless `#builtin`** whose signature takes a `$T: Type` parameter, when the

src/ir/lower/generic.zig

@@ -152,6 +152,16 @@ pub fn monomorphizeFunction(self: *Lowering, fd: *const ast.FnDecl, mangled_name
             const result = self.builder.callBuiltin(bid, &.{param0}, ret_ty);
             self.builder.ret(result, ret_ty);
         } else {
+            // Unknown bodiless #builtin: the name is not claimed by any
+            // recognizer (atomics/reflection are handled earlier in call.zig).
+            // Emitting `ensureTerminator(ret_ty)` here would synthesize a
+            // silent `constInt(0, ret_ty)` for a non-void return — a silent
+            // fallback default (issue 0144). Surface the failure loudly.
+            const span = if (fd.name_span.end != 0) fd.name_span else fd.body.span;
+            if (self.diagnostics) |d|
+                d.addFmt(.err, span, "unknown #builtin '{s}'", .{fd.name});
+            // Lowering has already failed; close the block with a terminator
+            // so the IR stays well-formed for the rest of the pass.
             self.ensureTerminator(ret_ty);
         }
         self.builder.finalize();