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

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();