`____` has an empty middle between the two `__` pairs and does not match
`/^__.+__$/`; assert that (not a match) and add `__x__` as the shortest
real dunder form that does.
- Worker consumes PORTABLE_RESERVED_WORDS / RESERVED_ERROR_MEMBERS by
their seam names directly, dropping the local re-alias (symmetry with
the other two imported constants).
- Split the reserved-vs-duplicate diagnostics: a backend-owned global now
reports "reserved binding global", not the misleading "duplicate".
- DUNDER_MEMBER uses `__.+__` so a bare `__` (empty middle, not a real
CPython dunder) is not matched; add coverage.
- Worker misuse tests add `a$b` (second-char `$`) and `lambda` (Python
keyword) so the identifier narrowing and reserved-word adoption are
each pinned directly, not only transitively.
- Clarify the seam JSDoc (dunder-vs-explicit-set wording, Python backend
is a later stack PR) and record in the Agent Note the obligation to
widen RESERVED_BINDING_GLOBALS when the bootstrap seeds more globals.
Move the reserved-word, reserved-global, reserved-error-member, and
dunder exclusion sets from the worker backend up to the code-runtime
seam package, and narrow the portable identifier subset to drop the
JS-only `$`. Every backend now imports one contract so a binding
namespace list valid on one backend is valid on all.
Delivers only the seam extension and the worker's adoption; the Python
backend, py-types renderer, and Code Mode language dispatch are later
PRs in the stack that depend on these exports.
