mux: FMTS reads only our-phase units (AacsKeyMap::read_plan)

An FMTS forensic segment interleaves our device group's variant with a
foreign group's at the aligned-unit level. The mux was decrypting only
our phase but leaving the alternate (foreign) units in the buffer as
ciphertext, trusting the demux to 'drop untouched ciphertext cleanly'.
It doesn't: random 0x47 bytes at the 192-byte stride hit tracked PIDs,
mis-parse, and trip the demux's concealed-gap keyframe-resync — which
drops GOOD frames of ours around every segment (349 resyncs / ~6391
packets on Stand by Me, visible as playback flaws).

The map already knows which unit each LBA is and, for a forensic
segment, which phase is ours. AacsKeyMap::read_plan turns that into the
title's read plan: every default/CPS unit, plus inside a segment ONLY
our-phase units. The alternate units are never fetched, decrypted, or
handed to the demux — the demux sees one gapless our-variant stream.

- read_plan is general (single-CPS, multi-CPS, FMTS): a map with no
  Even/Odd range returns the extents unchanged, so DVD/CSS, single-CPS
  UHD and multi-CPS Blu-ray read byte-for-byte as before.
- Wired into build_iso_pipeline (the file-backed highway that muxes
  resumed ISOs). Producer re-anchors unit_base per extent, so per-unit
  segment reads stay unit-aligned and decrypt correctly.
- Extent gains PartialEq/Eq for the read_plan tests.

Tests: read_plan non-forensic unchanged; forensic omits exactly the
alternate units, kept units match the decrypt gate unit-for-unit. All
2314 lib tests pass; precommit green on 1.86.

Pending: end-to-end ISO re-mux validation (concealed gaps 349 -> ~0).

src/disc/mod.rs

@@ -457,7 +457,7 @@ pub(crate) fn chapter_name(i: usize) -> String {
 }
 
 /// A contiguous range of sectors on disc.
-#[derive(Debug, Clone, Copy)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub struct Extent {
     pub start_lba: u32,
     pub sector_count: u32,