mercury: remaining fast-path eligibility gaps

[boxerab]

bail census over grok-test-data (421 files) after #423 closed. each still sends the stream to classic, so #426 cannot retire classic until they move.

• HT and BYPASS code-block modes (Cmodes 0x40, 0x48, 0xc0, 0x01, 0x34): 22 files. mercury's packet parser reads one segment length per contribution and needs the part-15 / multi-segment length signalling, then the T1 shim needs the HT coder.
• COD/COC in tile-part headers (0xff52, 0xff53): 18 files. needs a per-tile coding style in the plan; today the header is rejected in comb_tile_part.
• Psot=0 (last tile-part runs to EOC): 6 files. the SOT walk needs the file length as the implied end.
• derived quantization (Sqcd style 1): 2 files. one LL step size scaled per band, not wired in the band delta table.
• PPM (packed packet headers in the main header): grok bails. mercury reads packet headers from the packet stream only.
• RGN (ROI upshift, main or tile-part header): grok bails, mercury rejects the tile-part marker. needs the max-shift descaling in the block decode.
• PPT and tile-part POC/RGN: rejected by mercury, classic keeps them.

also classic-only by decision in #423: component filter, asynchronous decode, transcode packet-length recording, part 2 transforms.

[boxerab]

progress:

• Psot=0: done in df7a54b. six corpus files moved to the fast path.
• derived quantization: done in df7a54b (grok already expands it per band, the handoff now says so). both corpus files with it are corrupt fuzzer inputs that fail later in the plan.
• COD/COC/QCD/QCC in tile-part headers: done in ae517a1. mercury parses the four markers in a tile's opening tile-part and resolves per-tile coding style and quantization; a tile changing the kernel or MCT flag still falls back. 13 of the 18 files moved to the fast path, the other 5 are corrupt streams that classic also fails or abandons (crash-0a/issue726 declare 97 tiles and carry one, issue399 tile 4 declares more bit planes than the band holds, issue775 has an HT tile COD and a short Psot, edf_c2_1103421 has a broken COC).

still open: HT and BYPASS code-block modes, PPM/PPT, RGN (main and tile-part), tile-part POC.