v0.13.23 — stop discarding the drive's SCSI sense data

Through the entire 0.13.x line, every CHECK CONDITION reply from the
drive (the standard way SCSI tells you why a sector failed) was being
collapsed into a synthetic status=0xFF, sense_key=0 transport-wedge
sentinel and the actual sense data was thrown away. Confirmed live on
the BU40N reading Dune 2 on 2026-04-27: drive returned host_status=0,
driver_status=8, status=2, exec_elapsed_ms=1416 on every bad sector
— a clean CHECK CONDITION carrying full sense data — and Disc::copy
was bailing on it as if the bridge had wedged.

Root cause: scsi/linux.rs's wedge check was
  `host_status != 0 || driver_status != 0`
SG's DRIVER_SENSE bit (0x08) is set on every CHECK CONDITION reply
just to flag "sense buffer is populated" — it's not a transport
failure on its own. Pre-fix we conflated the two and silently lost
every drive-reported error reason. macOS and Windows backends had
the same shape: they extracted sense_key only, dropping ASC/ASCQ.

API restructure (clean separation):

  Error::ScsiError {
      opcode: u8,
      status: u8,                  // 0xFF = synthetic transport-failure
      sense: Option<ScsiSense>,    // None ⇔ no sense delivered
  }

  pub struct ScsiSense { sense_key: u8, asc: u8, ascq: u8 }
  impl ScsiSense {
      pub fn is_marginal(&self) -> bool       // keys 0/1/3/B
      pub fn is_medium_error(&self) -> bool
      pub fn is_hardware_error(&self) -> bool
      pub fn is_unit_attention(&self) -> bool
      pub fn is_data_protect(&self) -> bool
      pub fn is_not_ready(&self) -> bool
      pub fn is_illegal_request(&self) -> bool
      pub fn is_aborted_command(&self) -> bool
  }

  impl Error {
      pub fn scsi_sense(&self) -> Option<&ScsiSense>
      pub fn is_scsi_transport_failure(&self) -> bool
      pub fn is_marginal_read(&self) -> bool
  }

SCSI protocol constants (SCSI_STATUS_*, SENSE_KEY_*) moved from
error.rs to scsi/mod.rs where they belong alongside SCSI_INQUIRY,
SCSI_READ_10, etc. parse_sense replaces parse_sense_key (returns the
full triple, not just the key); inline tests now exercise ASC/ASCQ
extraction at the right offsets for both descriptor (0x72/0x73) and
fixed (0x70/0x71) sense formats.

Disc::copy + Disc::patch sense-aware dispatch:
  - marginal sense (MEDIUM ERROR / ABORTED COMMAND / RECOVERED ERROR
    / NO SENSE) → engage hysteresis (Block→Single, bpt=1)
  - non-marginal sense (HARDWARE / DATA PROTECT / UNIT ATTENTION /
    NOT READY / ILLEGAL REQUEST / transport failure / kernel
    IoError) → bail with full sense info preserved; caller (autorip)
    surfaces "physical replug" / "drive failing" / "media changed"

  Pre-fix: every CHECK CONDITION → 0xFF synthetic → Disc::copy bailed
  → bytes_good froze at the bad zone. The hysteresis from v0.13.22
  was correct but never got to run. This release unblocks it.

  Disc::patch's wedged_threshold (50 consecutive failures) stays as
  defense-in-depth for chains of marginal failures; a single
  non-marginal sense now short-circuits it.

New phase=bail trace event records the bail reason with the sense
triple. phase=transport_err remains for genuine bridge wedges /
kernel timeouts; phase=scsi_err carries the parsed sense_key, asc,
ascq for drive-reported errors.

All 350 tests pass. Clippy clean across all targets.

src/error.rs

@@ -155,10 +155,24 @@ pub enum Error {
     },
 
     // SCSI (4xxx)
+    /// SCSI command failed.
+    ///
+    /// `opcode` is the failing CDB byte 0. `status` is the raw SCSI
+    /// status byte: `0x02` = CHECK CONDITION (drive replied with sense
+    /// data), `0xFF` = libfreemkv-synthesised sentinel meaning "no SCSI
+    /// status delivered" (kernel timeout, USB bridge wedge, IOKit
+    /// service failure). `sense` carries the drive's SPC-4 sense triple
+    /// when the drive replied; `None` for transport-layer failures.
+    ///
+    /// Recommended dispatch (callers shouldn't pattern-match raw
+    /// fields):
+    ///   - [`Error::is_scsi_transport_failure`] — bail; bridge/transport wedge
+    ///   - [`Error::is_marginal_read`] — drive said this read was marginal; smaller block may recover
+    ///   - [`Error::scsi_sense`] — borrow the sense triple for finer routing ([`ScsiSense::is_medium_error`] etc.)
     ScsiError {
         opcode: u8,
         status: u8,
-        sense_key: u8,
+        sense: Option<crate::scsi::ScsiSense>,
     },
 
     // I/O (5xxx)
@@ -363,17 +377,26 @@ impl std::fmt::Display for Error {
             Error::ScsiError {
                 opcode,
                 status,
-                sense_key,
-            } => {
-                write!(
+                sense,
+            } => match sense {
+                Some(s) => write!(
                     f,
-                    "E{}: 0x{:02x}/0x{:02x}/0x{:02x}",
+                    "E{}: 0x{:02x}/0x{:02x}/0x{:02x}/0x{:02x}/0x{:02x}",
                     self.code(),
                     opcode,
                     status,
-                    sense_key
-                )
-            }
+                    s.sense_key,
+                    s.asc,
+                    s.ascq,
+                ),
+                None => write!(
+                    f,
+                    "E{}: 0x{:02x}/0x{:02x}",
+                    self.code(),
+                    opcode,
+                    status,
+                ),
+            },
             Error::IoError { source } => write!(f, "E{}: {}", self.code(), source),
             Error::DiscRead { sector } => write!(f, "E{}: {}", self.code(), sector),
             Error::Halted => write!(f, "E{}", self.code()),
@@ -438,6 +461,58 @@ impl From<Error> for std::io::Error {
 /// Convenience alias for `Result<T, Error>`.
 pub type Result<T> = std::result::Result<T, Error>;
 
+impl Error {
+    /// Borrow the drive-returned SPC-4 sense triple if this error is a
+    /// [`Error::ScsiError`] carrying sense data. `None` for any other
+    /// variant **and** for `ScsiError`s that represent a transport-layer
+    /// failure (where the device never delivered a SCSI status reply, so
+    /// no sense data exists).
+    pub fn scsi_sense(&self) -> Option<&crate::scsi::ScsiSense> {
+        match self {
+            Error::ScsiError {
+                sense: Some(s), ..
+            } => Some(s),
+            _ => None,
+        }
+    }
+
+    /// True if this is a [`Error::ScsiError`] representing a transport-layer
+    /// failure — kernel timeout, USB bridge wedge, IOKit service error.
+    /// The device never delivered a SCSI status reply, so there is no
+    /// sense data to inspect; retrying typically requires physical
+    /// intervention (replug).
+    pub fn is_scsi_transport_failure(&self) -> bool {
+        matches!(
+            self,
+            Error::ScsiError {
+                status: crate::scsi::SCSI_STATUS_TRANSPORT_FAILURE,
+                ..
+            }
+        )
+    }
+
+    /// True if the underlying SCSI failure is a *marginal read* — the
+    /// drive returned an error category in which smaller-granularity
+    /// retries can sometimes recover the data:
+    ///
+    ///   - MEDIUM ERROR (sense key 3) — canonical bad-sector signal
+    ///   - ABORTED COMMAND (sense key B) — transient; retry usually works
+    ///   - RECOVERED ERROR (sense key 1) / NO SENSE (sense key 0) — not
+    ///     classified as fatal; treat as recoverable
+    ///
+    /// Returns `false` for transport failures (no sense data delivered),
+    /// HARDWARE ERROR, DATA PROTECT, UNIT ATTENTION, NOT READY, ILLEGAL
+    /// REQUEST, BLANK CHECK, kernel `IoError`, and any non-SCSI variant.
+    /// Caller-agnostic predicate — describes a property of the *error*,
+    /// not what one specific call site should do with it. Used by
+    /// `Disc::copy`'s hysteresis dispatch.
+    pub fn is_marginal_read(&self) -> bool {
+        self.scsi_sense()
+            .map(crate::scsi::ScsiSense::is_marginal)
+            .unwrap_or(false)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     //! Smoke tests for the error code → variant mapping. Each new variant