docs(protocol): document protocol 28-32 interop matrix (#1908)

[oferchen]

Summary

Adds docs/audits/protocol-28-32-interop-matrix.md, a wire-and-symbol
audit cataloguing per-version (28, 29, 30, 31, 32) support for:

• File list encoding (varint vs 4-byte LE), NDX wire format, binary
  vs ASCII handshake, CompatibilityFlags exchange.
• Capability characters (-e.LsfxCIvu), the CAPABILITY_MAPPINGS
  table, INC_RECURSE asymmetry, pre-release 'V' byte-encoded path.
• Filter wire prefixes (old XFLG_OLD_PREFIXES vs modern multi-char),
  s/r/p modifier gating.
• Hardlink dev/ino encoding (legacy XMIT_SAME_DEV_PRE30 /
  XMIT_RDEV_MINOR_8_PRE30 vs proto-30 inline), iflags, multi-phase.
• ACL/xattr wire fields, NDX_DEL_STATS (proto >= 31), zstd/lz4
  vstring negotiation, MD4 -> MD5 -> XXH3/XXH128 boundary, batch
  format flag-by-flag protocol gating.
• CI coverage (versions in tools/ci/run_interop.sh:53,
  per-version test files), known failures from
  tools/ci/known_failures.conf, and the 3.0.9 protocol-number
  discrepancy still open from PR docs(daemon): tcpdump audit of pull-direction filter wire (#1697) #3503.

Closes #1908.

Guarantees

• Docs only. No code, configuration, or other documentation changed.
• Cites real symbols using path:line format, verified against master
  83c8aa41 (capabilities.rs, compatibility/flags.rs, codec/ndx,
  filters/wire.rs, transfer/setup/capability.rs, batch/format/flags.rs).
• Matches the existing docs/audits/ audit style (TL;DR, scope,
  numbered sections, references block) - see
  docs/audits/iouring-pbuf-ring.md and
  docs/audits/zstd-batch-compatibility.md.
• Conventional-prefix docs(protocol): on commit and PR title;
  prose uses hyphens (no em-dashes).