Route by queue, not by directory: stop moving files between stages

A file is now written once into spool/ and stays there for its whole time in
flight. Stages 1 and 3 hand work on by enqueueing the same Job reference, so
job.path is constant from intake until commit. The three inter-stage renames
(spool->known, spool->unknown, unknown->text) are gone, along with the
known/, unknown/ and text/ directories and their FS_*_DIR settings.

Terminal moves stay: done/, text_done/, binary/ and failed/ still receive the
file, and binary/ in particular must, since it is the corpus the offline
stage-5 discovery sweep reads.

Measured by bin/bench_stage1.jl (2000 x 64 KiB, min of 5): the removed rename
cost 11.6 us per file, the equal of the classifier itself. One stage-1 worker
goes from ~28.6 us/file (~35k files/s) to 11.70 us (85.4k files/s); 16 workers
reach 333k files/s. What is left is classify 10.30 us, the queue handoff
0.12 us, and the disabled @debug lines 0.29 us.

The stage a file has reached now lives only in the queue holding its
reference, and the queues are in-process, so a crash loses it: everything in
spool/ replays from stage 1. That is safe rather than merely tolerable —
classification and the UTF-8 sniff are pure functions of the file's bytes and
the terminal commits rename with force=true, so a replayed file lands where it
would have landed and overwrites its own sidecar. The per-stage directories
were standing in for a durable queue, and charging every file a rename per
stage to do it; src/queue.jl already defines the seam where a broker-backed
JobQueue restores exact resume properly.

Recovery had to change to match. All leftovers now funnel onto the single
stage-1 queue, so the old non-blocking recover_dir! would have capped a
4000-file recovery at 1000 and abandoned the rest. It now blocks on a full
queue, and run() spawns the worker pools before recovering so the consumers
drain it as we fill; reset_metrics! moves above the spawn accordingly.
enqueue_blocking! takes stats::Union{StageStats,Nothing} so recovery reuses
the never-drop retry without charging its wait to a stage's blocked_ns, which
would drive /stats utilization negative.

Tests assert the new invariant positively (the file stays put, the routed
reference is unchanged, no intermediate directory appears) and cover recovery
of more files than the queue can hold. Verified end to end against a real
server: 40 leftovers, stage-1 capacity 3, all recovered and drained to
text_done/ with spool/ and failed/ empty.

src/language.jl

@@ -1,7 +1,7 @@
 # Stage-4 language enrichment for text files.
 #
-# A file that stage-3 sorted into `text/` is human-readable, but we don't yet
-# know *what* it is. This stage answers two questions and records them in a
+# A file that stage-3 sorted as text is human-readable, but we don't yet know
+# *what* it is. This stage answers two questions and records them in a
 # `.meta.json` sidecar, exactly like the stage-2 known-file enrichment:
 #
 #   * natural language  — via Languages.jl's `LanguageDetector` (a Julia port of