sector: generic recovery seam; FMTS forensic segments as decrypt loss

Replace the AACS-specific inline key-fetch in the decrypt decorator with
a scheme-neutral recovery seam: the input stream (L3) installs a Recover
closure (none / AACS key-fetch) and the decorator (L2) runs it at the
single decrypt-miss point. FMTS (AACS 2.1) forensic-segment units that no
key opens are just undecryptable units, concealed and counted as ordinary
decrypt loss with no FMTS-specific branch ("a loss is a loss"), so the
separate bytes_undecryptable bucket collapses into one loss count.

- sector/recovery.rs: the seam (MissOutcome, none/key_fetch factories),
  naming no encryption scheme in its type.
- FMTS: segment routing primitives + BYPASS_FMTS_KEY, and an upfront
  ensure_forensic_segments_decryptable gate (Error::FmtsKeyMissing) in
  the mux input path, parallel to the unit-key gate.
- CSS descramble/rekey moves from decrypt_sectors into
  css::descramble_region: CSS self-recovers from the data itself, so it
  stays OFF the seam (which is only for external inputs).
- disc/mod.rs also: main-title selection aligned to largest physical
  size; is_regular read from the open file handle, not metadata(path),
  fixing a swallowed sync_all on a fresh-rip ISO. decrypt_threads()
  resolved once via OnceLock off the per-buffer hot path.

src/css/lfsr.rs

@@ -24,13 +24,12 @@ use super::tables::{TAB1, TAB2, TAB3, TAB4, TAB5};
 /// hierarchy, not the content cipher). Bytes 0x80..0x800 are recovered with
 /// `*p = TAB1[*p] ^ (i_t5 & 0xff)`.
 ///
-/// The scramble flag at byte 0x14 (bits 4-5) indicates encryption. Like
-/// libdvdcss, the flag byte is NOT modified here — the caller treats a
-/// nonzero `sector[0x14] & 0x30` as "needs unscrambling" and the descramble
-/// is its own inverse, so re-running it on plaintext would re-scramble.
-/// (freemkv historically cleared the flag; we keep clearing it so callers
-/// and the existing tests can distinguish a descrambled sector. This does
-/// not affect the recovered body.)
+/// The scramble flag at byte 0x14 (bits 4-5) indicates encryption. This
+/// descrambler CLEARS that flag after unscrambling, so a descrambled sector
+/// reads as `sector[0x14] & 0x30 == 0`; callers and the tests use that to tell
+/// it from ciphertext, and re-running descramble on an already-cleared sector
+/// is a no-op (the flag guard below skips it). Clearing does not affect the
+/// recovered body.
 ///
 /// No-op (returns without modifying `sector`) in two cases:
 /// - `sector.len() < 2048`: the encrypted region (0x80..0x800) is not