v0.13.13 — telemetry: tracing instrumentation in SgIoTransport + Disc::copy

v0.13.12 shipped the async fd_recovery design but a live test on Dune 2
showed Pass 1 sat for 14 minutes with bytes_good=0 — the inner loop iterates
(throttled on_progress log fires every 78s) but each iteration evidently
takes ~60s instead of the microseconds the design promises on fast-fail.
Without trace-level telemetry at the SCSI + Disc::copy boundaries we
can't tell where the time goes.

This release is instrumentation only — no behavior change.

- New dep: tracing 0.1. Per CLAUDE.md, debug/trace logging is allowed in
  libfreemkv (the no-English rule applies to errors). Consumers wire a
  tracing subscriber.
- SgIoTransport::execute (Linux): trace at every state transition (entry,
  recovery_swap_ok, recovery_pending, write_ok / write_err, poll_done,
  timeout_spawn_recovery, scsi_err, read_err, ok). Each event includes
  opcode + elapsed timing. The bg recovery thread also traces close_ms +
  open_ms so we can see if the kernel really takes 60s to close+open on a
  wedged Initio bridge.
- Disc::copy: trace at copy_start, outer_loop, region_enter, every 100
  inner-loop iterations (iter_progress with pos/region_end/skip_size/
  bytes_good/read_ok_count/read_err_count/last_read_ms/copy_elapsed_ms),
  copy_done.
- All trace events use targets `freemkv::scsi` and `freemkv::disc` so
  consumers can filter by subsystem (e.g. autorip /api/debug?q=freemkv::scsi).

Next: run the live test on Dune 2 again, read the autorip JSONL log,
diagnose why each iter is slow, fix the actual bug.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "libfreemkv"
-version = "0.13.12"
+version = "0.13.13"
 edition = "2024"
 rust-version = "1.86"
 license = "AGPL-3.0-only"
@@ -24,6 +24,10 @@ rand = "0.8"
 cmac = "0.7"
 zip = { version = "2", default-features = false, features = ["deflate"] }
 base64 = "0.22.1"
+# Trace-level instrumentation for Disc::copy + SgIoTransport::execute. Permitted
+# under CLAUDE.md ("Acceptable strings: debug/trace logging"). Consumers (autorip)
+# wire a tracing subscriber and pipe events into the JSONL debug log.
+tracing = "0.1"
 
 [target.'cfg(target_os = "linux")'.dependencies]
 libc = "0.2"