fix: align the Linux fsync error with macOS, and clear three stale docs

Round 9 findings, triaged and verified against the pinned tree.

writeback_file: a bounded-fsync WorkerLost returned bare ErrorKind::Other
on Linux where macOS returns EIO. Round 8 fixed the Linux arm to return
Err at all — the right fix — but stopped short of matching the value, so
a consumer distinguishing timeout / halt / lost-worker had nothing to
branch on for the third case on one platform. Now EIO on both.

Three doc comments described the pre-fix behaviour, one of them for
longer than the bug existed:

  linux.rs durable_sync still said "all three fallbacks return Ok(())"
  mod.rs sync_all still said Linux silently swallows fsync failures and
    callers must not treat Ok(()) as a durability barrier
  mod.rs SequentialSink::finish repeated the same caveat

All three now say what the code does: a bounded-fsync failure is an Err
on every platform, so Ok(()) IS a durability barrier. A doc that
describes a fixed bug is worse than no doc — it tells a caller to write
a workaround for something that no longer exists.

au_assembly: discard_gap_before duplicated drop_marks_before's
mark-retirement body verbatim and added one statement. Mine, from
earlier today. It now calls it. Two copies of the same retirement loop
is exactly how the two call sites would drift back together.

clpi: ClpiStream's audio_format / audio_rate / video_format / video_rate
are decoded from untrusted on-disc bytes on every parse and read by
nothing. The identically-named fields consumed in disc/bluray.rs belong
to mpls::StreamEntry, not to this struct — checked, because an earlier
round wrongly called a live function dead. Deleted, along with the seven
test assertions that pinned them; the tests that pin pid, coding_type
and language remain. Also removed a section-header comment orphaned by
the get_extents deletion, describing a fixture that no longer exists.

src/mux/au_assembly.rs

@@ -535,12 +535,7 @@ impl AuAssembler {
     /// no longer exist, and the AU that eventually emits takes its PTS from the
     /// fragment that actually opened it.
     fn discard_gap_before(&mut self, off: u64) {
-        while self.marks.front().is_some_and(|m| m.off < off) {
-            self.marks.pop_front();
-        }
-        while self.disc_marks.front().is_some_and(|&o| o < off) {
-            self.disc_marks.pop_front();
-        }
+        self.drop_marks_before(off);
         self.pending_gap = true;
     }
 }