backlog#1052 S3, third slice — the hard blocker for a second embedded
server's service stage. GLOBAL_BUCKET_METADATA_SYS was a process-global
OnceLock whose init ran .set().unwrap(): a second instance's service
startup panicked the whole process. The system it guards is inherently
per-store (it holds the store handle and that store's bucket→metadata
cache), so it now lives on the store's own InstanceContext:
- init_bucket_metadata_sys writes the cell of the store it was handed
(api.ctx) — no signature change — and keeps the double-init fail-fast,
now scoped to the instance (assert on the per-context set). The
dist-erasure check for the refresh loop reads the same context.
- get_global_bucket_metadata_sys / the crate-private getter behind the
~20 get_*_config helpers resolve through the current_ctx() facade —
the published store's context, or bootstrap before that — so every
existing reader keeps its exact single-instance behavior.
- New acceptance test: two real stores in one process each initialize
their own metadata system (the old global cell panicked right there),
plus a shared builder extracted from the store-graph test.
201 bucket/metadata regression tests green.
