v0.13.22 — replace bisect-on-fail with hysteresis Block↔Single

The v0.13.21 bisect-on-fail recovery was correct (100% of recoverable
sectors picked up) but slow on dense damage clusters. Live test on
Dune 2 v0.13.21 burned ~30 s per damaged 60-block — paying a ~5 s
kernel ABORT/timeout at every level of a log₂(60) ≈ 6 deep DFS, on
the failing branch each time.

Replaced with a two-state hysteresis machine in Disc::copy:

  Block(batch):
    read(batch) ok    → write, advance, stay Block
    read(batch) fail  → switch to Single, retry SAME range at bpt=1

  Single:
    read(1) ok    → write, consecutive_good++
                    if consecutive_good >= BPT1_EXIT_THRESHOLD:
                      switch to Block, reset counter
    read(1) fail  → mark NonTrimmed, consecutive_good = 0

BPT1_EXIT_THRESHOLD = 10_000 sectors (= 20 MB clean run). Calibrated
from the 2026-04-26 BU40N empirical probe data; tunable.

Per-block math on a damaged 60-block with 1 truly bad sector:

  Bisect      (v0.13.21): ~30 s  (5 s × 6 levels)
  Hysteresis  (v0.13.22): ~10 s  (5 s bpt=batch fail
                                  + 59 × 1 ms good
                                  + 1 × 5 s bad)

Inside a damaged cluster spanning many 60-blocks the win compounds:
hysteresis pays the bpt=batch fail cost ONCE on entry, then stays at
bpt=1 across the cluster; bisection re-paid it every 60 sectors. For
Dune 2's ~1248-sector boundary cluster that's ~21 fewer 5-sec
kernel timeouts ≈ 100 s saved per pass.

Telemetry: new phase=mode_change trace event with from, to, lba, and
consecutive_good. Replaces v0.13.21's phase=bisect. Worklist DFS is
gone — single iterative for s in 0..count on the failure path.

Test rename, same fixture and same 100% recovery expectation:
  test_disc_copy_bisect_recovers_via_single_sector_reads
  → test_disc_copy_hysteresis_recovers_via_single_sector_reads

Also adds DamageSeverity (Clean / Cosmetic / Moderate / Serious) +
classify_damage(bad_sectors, lost_ms), re-exported from libfreemkv,
so applications can render structured severity instead of formatting
their own from raw counters.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "libfreemkv"
-version = "0.13.21"
+version = "0.13.22"
 edition = "2024"
 rust-version = "1.86"
 license = "AGPL-3.0-only"