feat: prove the PNG round trip (IMG-PNG-2), and IMG-PNG-9 and IMG-PIX-1 with ancillary chunks

[ngngardner]

Summary

This proves three rows:

• IMG-PNG-2: proved. It is reworded, as the maintainer approved, to rasters with fewer than 2^29 samples.
  • png_roundtrip takes that bound as a U32 a8 equal to 8·h·w, and splits on the scanline byte count.
  • Up to 65535 bytes, it uses png_roundtrip_one from feat: lift the PNG pixel stage to decode_png, and prove the one-block PNG round trip #64.
  • Past that, both wide paths write the same file as spec.png over zlib.stored(65535, raw), with the right IDAT length and CRC:
    • colour type 6, through enc.seal.wide and enc.pour;
    • colour type 2, through enc.wide.rgb and enc.rgb.go.
  • The arithmetic shows no size wraps: n ≤ 5·w·h and n + 5k + 6 ≤ 8·w·h.
• IMG-PNG-9: proved. png_walk_anc extends the chunk walk to ancillary chunks after IHDR, between PLTE and tRNS, after tRNS, and after the IDAT run. It also covers the tRNS-before-PLTE order the decoder accepts for colour type 2. Every other order is refused by the decoder, so the law covers every file decode_png accepts.
• IMG-PIX-1: proved. With the PNG side done, its "Left to prove" row is empty.
• SPEC's known-failures paragraph now reads "No row is known to be false." IMG-JPG-3 is the only row still pending.
• There are no src/ changes. The one-block proof's tail moves into rt.tail so the wide paths can reuse it.

Checks

• bend PROOF.bend prints All terms check.
  • On my machine, the proof check takes about 400 s against main's 270 s. Most of the increase comes from the IHDR CRC over a symbolic size.
• bolt v1.7.0 reports 0 errors; CI runs v1.8.0.
• Pillow reports 0 hard failures in 40 cases.
• Probes: 224 identical, 0 differing, against a driver rebuilt from main.
• Each of these planted mutations fails the proof check:
  • NLEN taken from LEN in enc.pour;
  • a wrong Adler sum in enc.rgb.go;
  • enc.nblk off by one;
  • the ancillary bit read from the wrong position;
  • PLTE refused after tRNS.

🤖 Generated with Claude Code

https://claude.ai/code/session_01A1bVZYFbhKkn2BKHKthcVP

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