fix(execution): MatchResult bincode round-trip — serialize outcome as Option (#135)

[joaquinbejar]

Summary

Fixes #135 — MatchResult could not round-trip bincode since 0.9.0.

The decode-time validation from #117 deserializes through MatchResultWire,
whose outcome is Option<MatchOutcome> (to accept legacy payloads), but
the Serialize derive still emitted a bare MatchOutcome. JSON tolerated
the asymmetry; bincode decodes positionally and read the enum variant index
where the Option tag was expected:

UnexpectedVariant { type_name: "Option<T>", allowed: Range { min: 0, max: 1 }, found: 2 }

This blocks OrderBook-rs's bincode feature (NATS zero-copy path) from
bumping to 0.9 (joaquinbejar/OrderBook-rs#205).

Changes

• outcome is serialized via Serializer::serialize_some, making the
  emitted shape symmetric with the wire struct. JSON payload is
  byte-identical (serde flattens Some); bincode encode/decode now agree.
• Regression coverage: a bincode leg on the existing
  valid_result_round_trips_both_encodings property test, a deterministic
  bincode_round_trip_is_symmetric_issue_135 shape test, and a JSON guard
  asserting the payload still carries the bare "outcome" value.
• New dev-dependency: bincode = "2.0" (serde + std features only,
  dev-only) to drive the positional-format regression.
• Version 0.9.0 → 0.9.1 + CHANGELOG entry.

Compatibility

• JSON payloads: unchanged in both directions, any version.
• Bincode payloads written by 0.9.0: still do not decode — they never did
  (0.9.0 could not decode its own output), so no working payload breaks.
• Pre-0.9 bincode payloads: unchanged (positional formats cannot express
  the absent trailing field either way).

Test plan

• cargo test --all-features — 528 tests green.
• make pre-push clean (clippy -D warnings, fmt, doc -D warnings).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/execution/match_result.rs	94.01% <100.00%> (+1.10%)	⬆️

... and 1 file with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.