- Worker consumes PORTABLE_RESERVED_WORDS / RESERVED_ERROR_MEMBERS by
their seam names directly, dropping the local re-alias (symmetry with
the other two imported constants).
- Split the reserved-vs-duplicate diagnostics: a backend-owned global now
reports "reserved binding global", not the misleading "duplicate".
- DUNDER_MEMBER uses `__.+__` so a bare `__` (empty middle, not a real
CPython dunder) is not matched; add coverage.
- Worker misuse tests add `a$b` (second-char `$`) and `lambda` (Python
keyword) so the identifier narrowing and reserved-word adoption are
each pinned directly, not only transitively.
- Clarify the seam JSDoc (dunder-vs-explicit-set wording, Python backend
is a later stack PR) and record in the Agent Note the obligation to
widen RESERVED_BINDING_GLOBALS when the bootstrap seeds more globals.