feat(disc): read a SACD image's stereo area, measured against two real discs

Adds an ISO 9660 walk (no mounting, no privileges) and a Scarlet Book parser
for the stereo audio area, plus the `DsdDemuxer` adapter that lets every DSD
delivery the player already has — PCM, DoP, native — play a disc image.

The parser exists because the previous account of this format, written in this
repo, was WRONG: it said the per-sector header was a fixed 8 bytes, derived
from looking at one sector. Every structure below was re-derived by a CENSUS —
3 066 893 sectors across both of the owner's Rheingold discs, zero exceptions —
and each claim is falsifiable rather than asserted:

  - byte 0 carries packet_count = (b>>5)&7 and frame_info_count = (b>>2)&7,
    so the header is 1 + pc*2 + fic*3 and takes the values 5, 8 or 10;
  - packet descriptors are BIG-endian, and the length field is ELEVEN bits,
    not the twelve every published layout shows. The first descriptor on both
    discs is 0x181B, whose low twelve bits are 2075 — larger than the sector
    it lives in. With eleven bits it is 27, and 5 + 27 + 2016 = 2048;
  - the proof of the whole decoding is the sum rule
    `header + SUM(packet lengths) == 2048`, which the parser CHECKS on every
    sector rather than trusting;
  - the area is uncompressed, proven from the data and not from a flag: the
    audio-packet bytes divide by 9408 (one uncompressed DSD64 stereo frame)
    exactly — 350 495 frames on Disc 1, 306 696 on Disc 2. DST is
    variable-length; those could not be integers.

Track geometry comes from SACDTRL1/TRL2, tied together by an identity that
holds for 44 of 44 tracks: start_lsn == area_start + start_time * 350. The
ISO 9660 extents mirror SACDTRL1 byte for byte, so the one-sector overlap
between consecutive tracks is the disc's own accounting, not a bug to fix.

The channel layout inside a frame was the last unknown, and three statistical
probes could not settle it — they were measuring the DSD noise floor. What
settled it was physics: converted with the real filter, block-per-channel puts
23 % of its energy above the audible band while byte-interleaving puts 0.2 %
there. A correctly decoded 1-bit stream MUST carry that rising noise floor,
and no filter puts it back once scrambling has folded it away.

Also in this commit, from the same measurements: `2C_TAREA.2CH` on Disc 1 is
recorded in FOUR extents, so `IsoImage::find` REFUSES a multi-extent file
rather than returning its first quarter as if it were the whole thing.

cc

crates/qbz-disc/Cargo.toml

@@ -6,6 +6,8 @@ edition.workspace = true
 [dependencies]
 log = { workspace = true }
 thiserror = { workspace = true }
+sha1 = "0.10"
+base64 = { workspace = true }
 
 [target.'cfg(target_os = "linux")'.dependencies]
 libc = "0.2"