docs(design): zsync seq-match heuristic semantics

[oferchen]

Design note for insertion point #2 of the zsync-inspired matching pipeline (issue #2064): the seq-match heuristic.

Summary

• Maps zsync's next_match/onlyone=1 shortcut (rsum.c:190, 262, 352-356) onto oc-rsync's existing want_i adjacent-block hint surface in crates/match/src/generator.rs (lines 103, 177-187, 221-225, 259-273) and DeltaSignatureIndex::check_block_match_slices (crates/match/src/index/mod.rs:229-251).
• Pins the duplicate-block tie-breaker rule, the misprediction cost bound (one extra strong-checksum verify worst case), and the benchmark plan binding for Ensure --ignore-times forces a rewrite #2067.

Sections

• TL;DR
• Background: what zsync's seq_matches does
• Mapping to oc-rsync architecture (data flow, when the hint fires vs falls back, when the hint clears)
• Wire-compat invariant (COPY-token byte equality, duplicate-block tie-breaker)
• Misprediction cost analysis
• Benchmark plan binding (Ensure --ignore-times forces a rewrite #2067) with corpus matrix and expected speedup band
• Wire-compat restatement (eight-layer checklist)
• Cleanup PR contract (Refactor CLI out-format tests into modules #2084-Split rolling checksum tests into dedicated modules #2086): no CLI flag, no protocol crate change, golden bytes stay green

Wire-compat

• No code changes. Doc-only.
• No CLI flag.
• No protocol crate change.
• COPY-token output stays byte-identical to the table-probe baseline.
• Golden byte tests in crates/protocol/tests/golden/ and the upstream interop matrix (tools/ci/run_interop.sh against 3.0.9 / 3.1.3 / 3.4.1) stay green.

Test plan

• No source files changed; only docs/design/zsync-seq-match.md is added.
• Verified git log origin/master..HEAD lists exactly one commit and one file.
• CI checks: fmt+clippy, nextest (stable), Windows, macOS, Linux musl - expected to pass since no code is touched.

Refs

• Parent design: docs/design/zsync-inspired-matching.md (PR feat(fast_io): cache PBUF_RING probe and surface in --version #3635 / commit 48b2411).
• Issue: Fix quick-check skip logic for ignore-times parity #2064 (this design); follow-ups Stabilize run_client delete test timestamps #2065 (impl), Fix local delta copy when identical timestamps #2066 (golden COPY-token equality test), Ensure --ignore-times forces a rewrite #2067 (benchmark), Refactor CLI out-format tests into modules #2084-Split rolling checksum tests into dedicated modules #2086 (cleanup).