audit: drop dead DTS marks cap, lazy passthrough buf, doc corrections

Round-9 findings from the 10-phase release audit (no HIGH):

- Remove the MAX_PTS_MARKS backstop and its tautological test: an empty
  DTS PES returns before recording a mark, and a non-empty run is already
  bounded by the MAX_AU_BYTES buffer clear (which clears pts_marks) — so
  the deque cannot grow unbounded and the cap was dead code.
- AuAssembler::for_codec no longer reserves 256 KiB for a Passthrough
  stream (audio/subtitle, and every TS/BD stream) whose buf is never
  written; only the reassembling modes reserve.
- Correct the scan comment that claimed region is computed (it is a
  Region-free stub until region detection lands) and drop a public-repo
  reference to internal "private refactor notes" in the mkb module doc.

src/disc/mod.rs

@@ -1950,8 +1950,10 @@ impl Disc {
         crate::labels::apply(reader, &udf_fs, &mut titles);
         crate::labels::fill_defaults(&mut titles);
 
-        // 5. Format (AACS MKB generation → BD/UHD/FMTS; tree → HD-DVD/DVD),
-        //    layers, region.
+        // 5. Format (AACS MKB generation → BD/UHD/FMTS; tree → HD-DVD/DVD) and
+        //    layers. Region coding is not yet decoded from the disc — every disc
+        //    reports Region-free for now (correct for all UHD; a stub for
+        //    region-locked BD/DVD until region detection is implemented).
         let format = Self::detect_disc_format(reader, &udf_fs, &titles);
         let layers = if capacity > 24_000_000 { 2 } else { 1 };
         let region = DiscRegion::Free;