Async cwd validation dedupes by exact path but had no concurrency bound, and a
dead UNC share answers `stat` in ~21s while holding one of libuv's 4 default fs
threads. Four distinct paths on one unreachable server therefore starved every
other async fs read in the daemon — including the cold-restore history replay
running alongside them — which moves the head-of-line stall #14848 removed from
the event loop into the thread pool.
Adds a per-route lane of 2, reusing PrioritySemaphore and matching the per-distro
lane in rate-limits/auth-filesystem-operation.ts. Keyed by the host that has to
answer (WSL distro, or the `\\server` prefix) so many dead subdirectories of one
share fold into one lane. Local-disk paths bypass the lane entirely: a global cap
would queue a healthy local spawn behind a dead share.
Moves PrioritySemaphore to src/shared. It has no imports, and reaching into
src/main/daemon from src/main/providers inverted the dependency direction that
already runs daemon -> providers.
Note this bounds pool occupancy, which cancellation cannot: an aborted `stat`
still holds its libuv thread until the OS returns.
STA-4543
Neil