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/mux/au_assembly.rs

@@ -142,7 +142,13 @@ impl AuAssembler {
         };
         Self {
             mode,
-            buf: Vec::with_capacity(256 * 1024),
+            // Passthrough never writes `buf` (one fragment → one unit); only the
+            // reassembling modes need reserve. Avoids ~256 KiB per audio/subtitle
+            // stream (and every TS/BD stream, which never feeds the assembler).
+            buf: match mode {
+                Mode::Passthrough => Vec::new(),
+                _ => Vec::with_capacity(256 * 1024),
+            },
             base: 0,
             marks: VecDeque::new(),
             disc_marks: VecDeque::new(),