unified read-error handler + pass N size-aware skip

New disc/read_error.rs as the single entry point all read failures flow
through. Handler classifies the error, updates the in-flight context
(damage window, retry budgets, jump multiplier), and returns a
ReadAction the caller dispatches on. Pass 1 (sweep) refactored to use
it; ~340 lines of nested if/else collapsed into ~120 lines of action
dispatch. Adding a new error class = one match arm. Logging is in one
place. Bisect inner failures don't poison the damage window. Jump
multiplier capped at 64 (max 1 GB jump for batch=32 — observed prior
unbounded behavior produce a single 56 GB jump on a wedged drive).

Pass N (patch) damage_skip is now size-aware: each skip is capped at
range_remaining/4 rather than the absolute MB-scale escalation. The
old logic could leap over a 100-sector bad range that hides a 50-sector
good middle; size-aware convergence finds the good middles instead.

Tests in tests/pass_n_size_aware_skip.rs exercise the size-aware skip
against synthetic patterns (25-bad/50-good/25-bad and three good
middles in a row) and prove ≥98% of good middles are recovered.
Existing test test_disc_copy_marks_failed_ecc_blocks_as_nontrimmed
updated to reflect that MEDIUM_ERROR now triggers single-sector
bisect (which the BlockSizeFailingReader succeeds at).

tests/scsi_recovery.rs

@@ -9,7 +9,6 @@
 //! These tests require a real /dev/sg* device and are therefore #[ignore].
 
 use std::path::Path;
-use std::time::Duration;
 
 #[test]
 #[ignore]