v0.5.8: heap-free QR encoder + crash-dump QR on framebuffer

Real hardware has no serial console, so a kernel crash only leaves a
frozen screen.  Every dump_lines() report now also renders the crash
text as a scannable QR below the white-on-red band (photograph the
screen instead of relaying text).

- rsnix/src/qr.rs: from-scratch no_std/no_alloc byte-mode QR encoder
  (EC-L, v1..v40, .bss statics only — safe on the fault path):
  RS over GF(256), codeword interleave, zigzag placement, 8 masks with
  N1-N4 penalty auto-selection, format/version info.
- gop.rs: dump_qr() joins dump lines into a static buffer, truncates
  with '...' at the v40-L capacity, encodes, and renders centered
  black-on-white with a 4-module quiet zone.
- tools/qrcheck: host harness (embedds qr.rs) + qrcheck.py cross-check
  against segno's placement primitives.  Verified bit-exact: 12
  versions x 8 masks forced, plus auto-mask choice matches segno's
  penalty evaluation.  Caught 3 encoder bugs (multi-block interleave
  offsets, transposed version info, mask eval ordering).
- QEMU e2e: page-fault dump screendump decodes via zbarimg, content
  matches the serial log line-for-line.
- test-msg.sh and test-fault.sh both PASS; debug+release 0 warnings.

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