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/aacs/mkb.rs

@@ -2,10 +2,9 @@
 //!
 //! The MKB record format (framing walker, the `MkbRecord` view, record-body
 //! finders), the MKBType / AACS-generation classification, and MKB-file
-//! utilities (content length, trimming, version). Consolidated here from the
-//! former `keys.rs` / `variant.rs` so the one place that understands MKB bytes
-//! is `mkb`. A follow-up collapses the remaining duplicate finders (see the
-//! private refactor notes) — for now both dialects live here side by side.
+//! utilities (content length, trimming, version). Consolidated here so the one
+//! place that understands MKB bytes is `mkb`. Some duplicate record finders
+//! still live side by side pending a follow-up that collapses them.
 
 // ── MKB record types ([C] Chapter 3) ──────────────────────────────────────
 // The ONE canonical set. Every record-type comparison in the `aacs` module