Harden fixed-width band padding and regression coverage

[dutch2005]

Summary

• Zero-initialize each complete fixed-width protocol band and copy rows using the protocol stride.
• Split the legacy compression implementation into focused modules under 200 lines.
• Make warning-gated builds clean, including QPDL header bounds and checked swap-file I/O.
• Add fixed-width and variable-width protocol goldens, PPD characterization, C++98 compatibility, and C++23 ASan/UBSan tests.

Relationship

Follow-up to #23, which upstream already closed after the original ML-1520 fix and physical-printer confirmation. This hardening PR is based directly on current OpenPrinting master and does not depend on or modify #22.

Verification

• Exact head: c1ddf92
• Docker matrix: normal, no-thread, no-JBIG, ASan/UBSan
• Warnings are fatal in test builds; the raw LXC log has zero compiler diagnostics and zero sanitizer findings.
• ML-2010 A5 golden: ba867847558f3aaab66396cb6905ef0a8d1f6d12468b577c3c0ad3c545298e3c
• ML-1520 A5 golden: efd1192e314342a6b05ef901d445fa0dceaee408ffcdbf0bc76611b5ea03da37
• Checked page, band, and plane serialization rejects truncation and short I/O.
• Generated fixed-bandwidth PPD characterization passed standalone from a clean archive.
• The exact Git archive was tested in Docker inside LXC 9100.

Remaining external gate

A maintainer should decide whether fresh physical fixed-width printer testing is required before merge; this PR does not claim that new hardware test.

[dutch2005]

Exact-head verification for c1ddf92 is complete.

• Hosted workflow passed: https://github.com/dutch2005/splix/actions/runs/33906041215
• Exact Git archive SHA-256: �1549a3a69f72dd07c754b949142736fab0ffe6cf83cc28d8aa36c043a04a532
• Docker in LXC 9100: normal, no-thread, no-JBIG, and fail-closed ASan/UBSan all passed.
• Eight protocol golden checks passed across the matrix (ML-2010 and fixed-width ML-1520).
• Raw LXC and hosted logs contain zero compiler diagnostics and zero sanitizer findings.
• Checked page/band/plane serialization round-trip and truncation rejection passed.
• GitHub reports this head MERGEABLE with zero review threads.

The upstream PR currently has no attached check rollup because the successful workflow ran on the fork push. Fresh physical fixed-width printer testing and substantive maintainer review remain external merge gates.