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
A bodiless #builtin with a $T: Type parameter that no recognizer matches folds
to 0 (exit 0) instead of erroring — while a non-type-param #builtin link-errors
loudly. Discovered during the atomics stream (Atomic methods ran to 0 before
recognition existed). The reflection/type-arg lowering path defaults instead of
rejecting (REJECTED-PATTERNS silent-fallback class). Repro + investigation prompt
in the issue. Open (unpinned — not added to the suite, since the repro currently
exits 0 by the bug).
