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

@@ -0,0 +1 @@
+