* perf(ci): carry extension boundary artifacts on a Blacksmith sticky disk
The boundary lane spends ~113s of its 214s rebuilding plugin-sdk
declaration artifacts that an actions/cache was meant to carry — but
that cache is evicted so fast under the repo quota that even its prefix
restore never hits. Same-repo runs now keep the artifacts on a sticky
disk with a coarse toolchain key (source changes intentionally hit a
stale snapshot: restored mtimes predate the fresh checkout, so the
mtime-based freshness predicate always routes into the incremental
rebuild — a stale snapshot can never false-skip the compile). Fork PRs
keep the GitHub cache path and can never write repository-global
snapshots. Seed/restore round-trip verified locally with rsync -aR
layout mirroring.
* perf(ci): restore boundary artifacts for the lint lane too
check-lint's shard runner spawns the same plugin-sdk boundary artifact
prep the boundary lane snapshots (~72s of its wall clock); it now
mounts the shared sticky read-only-by-convention (only the boundary
lane seeds) and restores before the run step.
* fix(ci): make the lint boundary-sticky mount strictly read-only
Autoreview caught a first-hydration race: with commit if-missing, a
cold-key lint job racing the boundary lane could commit an empty,
marker-less snapshot that if-missing then never repairs. commit false
makes lint a pure reader; only the boundary lane can seed.
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