Audit round 1: playback order, silent title drops, image durability

Four fixes from the first audit round. Every finding was verified against a
pinned tree and read directly before being accepted.

resolve_vts_key sorted a VTS title-VOB extents largest-first. That is the 1.5.1
garbage bug, and it grew back in a new code path: the comment claimed it
"matched the scan heuristic", but that heuristic WAS the bug and had already
been fixed in decrypt_keys_for_title, which documents the rule (PLAYBACK ORDER,
never largest-cell-first) and pins it with a regression test. A CSS DVDs biggest
cell opens with a long clear run and crack_key shares one sector budget across
the extent list, so starting there can exhaust it without ever MEETING
scrambled data — and CSS recovers the key from scrambled data itself. The crack
then returns None, the caller falls back to the disc-wide key, and every VOB in
that VTS is descrambled wrongly: corrupt PES behind an intact header, written
out as a complete extract at exit 0.

parse_pgcit dropped titles silently in THREE places — an unparseable PGC, an
out-of-range PGC index, and a truncated entry table. The finder caught one; the
other two turned up on reading the function. parse_vmg already counts and warns
per skipped title SET for exactly this reason, and this was the last place a
disc could quietly report fewer titles than it has.

write_image called flush() and returned Ok. flush() only pushes bytes into the
page cache and promises nothing about durability, so a 6-90 GB image could be
reported complete while still unwritten — a crash or an unmounted volume then
leaves a truncated file the caller was told was finished. Now into_inner (so a
buffered-write error surfaces instead of being dropped by BufWriter::drop)
followed by sync_all.

timeline used abs() on a saturating_sub result. Every other comparison in that
module is saturating because the timestamps come off a disc and are not
trusted; abs() panics on i64::MIN, which saturating_sub can produce.

src/mux/timeline.rs

@@ -264,7 +264,13 @@ impl SeamPlan {
             // the end of the list.
             let stepped_back = pos.last_raw_ns.is_some_and(|last| raw_ns < last)
                 && if has_reorder {
-                    (raw_ns.saturating_sub(next_in)).abs() <= CLIP_START_TOLERANCE_NS
+                    // saturating_abs, not abs: every other comparison in this
+                    // module is saturating because these timestamps come off a
+                    // disc and are not trusted. `abs()` is the one exception
+                    // and it panics on i64::MIN — which saturating_sub can
+                    // produce exactly — taking down the mux thread on one bad
+                    // frame instead of comparing false like its neighbours.
+                    (raw_ns.saturating_sub(next_in)).saturating_abs() <= CLIP_START_TOLERANCE_NS
                 } else {
                     raw_ns >= next_in.saturating_sub(CLIP_START_TOLERANCE_NS)
                         && raw_ns <= self.clips[clip + 1].out_ns