DVD vob_start absolute rebase + rc.5.3 audit fixes

- ifo.rs: rebase VTS title VOBS to absolute disc LBA (file_start_lba +
  vtstt_vobs); fixes DVD rips opening on the menu region instead of the
  movie (e.g. SOTL). Adds absolute-placement regression test.
- aacs/boil.rs: add mk_from_pk primitive (PK -> MK via MKB walk).
- dvdnav/: nav-VM command decoder + start-cell resolver seam, parked
  behind USE_NAV_RESOLVER (kept compiled, never executed).
- mux: FVI src.byte within-sector per spec; Unknown colour -> CICP
  unspecified (2,2,2,1); demux clear PCS -> NORMAL; ts.rs feed() base
  reset + boundary provenance fix.
- Assorted audit fixes (doc/comment/test accuracy) across the crate.

src/dvdnav/mod.rs

@@ -0,0 +1,49 @@
+//! DVD-Video navigation — read-only resolver for the **main-feature start
+//! point** (issue #40). Mirrors what a DVD player's nav VM resolves: First-Play
+//! → menu "Play" → title dispatch → the first cell of the feature, so the rip
+//! starts at the movie rather than at raw cell 0 (e.g. skipping a leading
+//! logo/warning segment when the disc's own navigation does).
+//!
+//! Byte layout follows the DVD-Video specification (VMGI/VTSI headers,
+//! PGC/cell tables, PCI/HLI button packets); the VM command decoder is
+//! verified against libdvdnav's decoder.
+//!
+//! Current contents: [`vmcmd`] — the VM command decoder (proven against the
+//! SOTL/Greenland test discs). The IFO/PCI parsing and the navigation executor
+//! that resolves the start cell build on top of this.
+
+pub mod vmcmd;
+
+use crate::sector::SectorSource;
+
+/// Resolve the feature title's **true start cell** (0-based index into the
+/// title PGC's cell list) by following the disc's own navigation — First-Play →
+/// menu "Play" → title dispatch — the way a player reaches the movie. This is
+/// what lets the rip begin at the feature instead of at raw cell 0 when the
+/// disc's nav enters the title past a leading logo/warning segment (e.g. a
+/// disc whose "Play" resolves to a later cell than cell 0).
+///
+/// Returns `None` when navigation cannot be resolved, so the caller falls back
+/// to the structural leading-cell filter (today's behaviour, ≈ cell 0 / 0:00).
+///
+/// TODO(#40): the IFO/PCI parsing + nav executor (built on [`vmcmd`]) land
+/// incrementally. Until the executor is complete this returns `None`, so wiring
+/// it in is behaviour-neutral; improvements to the resolver take effect here
+/// without touching the call site.
+pub fn resolve_feature_start(
+    reader: &mut dyn SectorSource,
+    udf: &crate::udf::UdfFs,
+    vtsn: u16,
+    vts_ttn: u16,
+) -> Option<usize> {
+    // `reader`/`udf` are the seam inputs the nav executor will consume to read
+    // VIDEO_TS.IFO + the VTS IFOs/menu VOBs. Reserved until that lands.
+    let _ = (reader, udf);
+    tracing::trace!(
+        target: "freemkv::dvdnav",
+        vtsn,
+        vts_ttn,
+        "nav start-cell resolver: unresolved — caller falls back to leading-cell filter"
+    );
+    None
+}