Demote stage-1 per-file logging to @debug; add stage-1 decomposition benchmark

bin/bench_stage1.jl takes handle_classify_job apart — filesize, read_features,
Lux.apply, classify, move_to, enqueue_blocking!, and the log lines — times each
in isolation, then times the real handler end to end under four loggers so the
parts can be checked against the whole.

It found that logging was stage 1's dominant cost: as @info the two per-file
lines cost ~71 us of the handler's ~118 us, roughly 6x the classifier (10.6 us)
and 6x the rename (11.6 us). Nearly all of it is ConsoleLogger formatting
(~64 us), not the FlushLogger's per-message flush (~8 us).

Demoting them to @debug takes stage 1 from 8.5k files/s to 35.3k files/s on one
worker (4.2x). The messages are still available with JULIA_DEBUG=FileServer,
which the benchmark also prices (133 us/file). What remains splits evenly
between the rename (11.7 us) and classify (10.7 us, itself 74% feature read),
so stage 1 is now filesystem-bound; its thread sweep peaks at ~4 workers.

src/worker.jl

@@ -18,6 +18,21 @@
 # this worker-to-worker handoff blocks.
 const ROUTE_ENQUEUE_RETRY_SECONDS = 0.05
 
+# Per-file logging in stage 1 is `@debug`, not `@info`, because it is the
+# stage's dominant cost. Measured by bin/bench_stage1.jl (2000 x 64 KiB files,
+# min of 5 trials): the two log lines cost ~71 µs of the ~118 µs
+# `handle_classify_job` spent per file — roughly 6x the classifier (10.6 µs) and
+# 6x the rename (11.6 µs). Nearly all of it is `ConsoleLogger` formatting
+# (~64 µs); the FlushLogger's per-message flush is only ~8 µs on top. Demoting
+# them takes stage 1 from ~8.5k files/s to ~35k files/s on one worker.
+#
+# `@debug` is compiled to a min-level check that doesn't evaluate its arguments,
+# so a disabled line costs ~0.15 µs rather than ~36 µs. The messages are still
+# there when wanted: run with `JULIA_DEBUG=FileServer` to get them back. Errors,
+# quarantines and lifecycle events stay at `@error`/`@info` — they are rare and
+# their cost doesn't scale with throughput. `GET /stats` (src/stats.jl) is the
+# per-file observability that survives, and it is counted, not formatted.
+
 """
     handle_classify_job(job, cfg, worker_id, known_queue, unknown_queue)
 
@@ -35,7 +50,7 @@ function handle_classify_job(job::Job, cfg::Config, worker_id::Int,
                              known_queue::JobQueue, unknown_queue::JobQueue,
                              stats::StageStats)
     classification = classify(CLASSIFIER[], job.path)
-    @info "classified file" worker=worker_id id=job.id name=job.original_name size=job.size classification=classification
+    @debug "classified file" worker=worker_id id=job.id name=job.original_name size=job.size classification=classification
 
     if classification === :known
         dest = move_to(cfg.known_dir, job)
@@ -43,13 +58,13 @@ function handle_classify_job(job::Job, cfg::Config, worker_id::Int,
         # known queue full → park and retry, don't drop (time charged to blocked_ns)
         enqueue_blocking!(known_queue, routed, stats;
                           retry_seconds = ROUTE_ENQUEUE_RETRY_SECONDS)
-        @info "routed to enrichment" worker=worker_id id=job.id dest=dest
+        @debug "routed to enrichment" worker=worker_id id=job.id dest=dest
     else
         dest = move_to(cfg.unknown_dir, job)
         routed = Job(job.id, job.original_name, dest, job.size, job.received_at)
         enqueue_blocking!(unknown_queue, routed, stats;
                           retry_seconds = ROUTE_ENQUEUE_RETRY_SECONDS)
-        @info "routed to content triage" worker=worker_id id=job.id dest=dest
+        @debug "routed to content triage" worker=worker_id id=job.id dest=dest
     end
     return nothing
 end