fix(usn): coalesce a backlog of apply ticks into one body rebuild

[githubrobbi]

Problem

Each USN apply does a full O(n) index rebuild (children CSR + trigram + ext index, ~600ms on a ~4M-record drive). When apply ticks fire faster than that drains — a small UFFS_USN_APPLY_INTERVAL_MS on a huge, busy volume — the applier channel backs up and the pipeline latency creeps to seconds, so a freshly renamed/deleted file can read stale until the backlog clears.

Surfaced by the verify harness pinning a 500ms apply interval (below the ~600ms rebuild cost) on a 3.76M-record C: — Round 2 (rename/delete) intermittently read pre-change state. The delete/rename logic was correct (the daemon logged deleted=true mapped=true / renamed=true mapped=true); the body just hadn't swapped before the search ran.

Fix (cheap robustness guard)

The applier coalesces a run of consecutive same-letter Apply messages into a single rebuild (coalesce_apply_run):

• greedily drains queued same-letter applies via try_recv, appending changes in FIFO order (create → delete → reuse-FRS sequences still apply correctly);
• stops at the first different-letter Apply / Save / Wrap and carries it forward intact — a Save is never merged (it keeps its cursor + persistence semantics).

This bounds the apply rate to the rebuild rate regardless of the interval — the system self-throttles instead of piling up rebuilds.

This is the cheap guard. The real fix for the per-apply O(n) cost is incremental base + delta + tombstone index maintenance (the CSR indexes are immutable/read-optimized, so it's an LSM-style two-tier redesign) — tracked as a separate, scoped project.

Tests

3 unit tests for coalesce_apply_run (merge same-letter run, stop at other letter, never swallow a Save). Full daemon suite + Windows-target clippy green.

Harness

scripts/windows/usn-verify.rs: POLL_SETTLE 3s→6s (the full pipeline is a couple seconds on a busy drive) and the apply pin 500→1500ms (above the rebuild cost), so the verification is deterministic.

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