9proc core becomes a backend of cloud9.fs; engine gains optional features

9proc's own fid table, walk loop and dir-read engine are replaced by
cloud9.fs.Server; the tree (static, vars, providers) is served through the
engine's Req/Reply contract with node ids that keep the old qid scheme.
Providers may answer later by returning error.Again (parked in the engine,
retried each step, Tflush -> EINTR); no new files are exposed.
Engine (backward compatible, all opt-in via Backend.features / Options):
create, remove, wstat, reference accounting for backends that count
handles, a salted fid index, name_capacity 0 (names from getattr),
Reply.ename for backend-chosen error text, Attr.path/version/atime.
Engine-level error strings and the 217-byte msize floor now apply to 9proc;
tests updated accordingly.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

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