Add proptest harness for the nine price-level invariants (#80)

[joaquinbejar]

Summary

Adds a property-test harness (proptest) covering the nine named price-level
invariants, driving a single PriceLevel through its public API with orders
built from the validated newtypes. User-approved dev-dependency.

Changes

• proptest added to [dev-dependencies] (default-features = false,
  features = ["std","bit-set"]); dev-only, no runtime dep.
• New tests/proptest target ([[test]], 256 cases, deterministic — fixed
  trade-id namespace + caller-supplied timestamps, no clock/RNG in the matcher).
• strategies.rs: maker_strategy (Standard / Iceberg / auto-reserve),
  book_strategy, taker_tif_strategy, all via newtypes at one level price.

Properties

1. Quantity conservation (+ counters == queue contents)
2. No zero-quantity orders rest or trade
3. FIFO / price-time priority (first-touch oldest-first; partial keeps front)
4. MatchResult agreement (is_complete ⇔ remaining==0; executed_quantity == Σ trade qty; executed_value; unique filled_order_ids)
5. Trade fields (maker≠taker, price==level, taker_side==maker_side.opposite())
6. IOC fills available + discards remainder (taker never rests)
7. FOK all-or-nothing (Filled vs Killed with zero trades + untouched queue)
8. Iceberg / reserve replenish (visible refreshed from hidden; totals conserved)
9. Snapshot round-trip + tampered checksum → ChecksumMismatch

Notes

• Three initial counterexamples were over-strict test assumptions, not engine
  bugs (confirmed via the minimized cases): a non-auto reserve correctly
  discards its stranded hidden when visible is consumed, and a replenished maker
  correctly loses time priority (re-queued at tail). Assertions adjusted; the
  generic book strategy uses auto_replenish: true.
• The committed properties.proptest-regressions records those seeds so they're
  retried in CI (they pass now). No production code changed.

Testing

cargo test: lib 408, proptest 9, integration 10, doctests 9 — 0 failed.
cargo clippy --all-targets --all-features -- -D warnings, cargo fmt --all --check, cargo build --release: clean.

Checklist

• proptest dev-only; deterministic; bounded case count
• All nine invariants covered, driving the public match_order (post-Decide and document taker TIF / PostOnly / MarketToLimit responsibility at the level #65)
• Counterexamples triaged (test-strategy, not engine bugs); no invariant weakened to hide a real defect
• No production change; no runtime dep

Closes #80

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[Copilot]

Pull request overview

This PR adds a deterministic proptest-based integration test target that exercises nine single-price-level invariants against PriceLevel via the public API, improving confidence in matching correctness (FIFO, conservation, TIF behaviors, snapshot integrity) without changing production code.

Changes:

• Add proptest as a dev-dependency and register a dedicated [[test]] target (tests/proptest/mod.rs).
• Introduce shared generation strategies for makers/books/taker TIF (tests/proptest/strategies.rs).
• Implement nine property tests plus persisted regression seeds (tests/proptest/properties.rs, tests/proptest/properties.proptest-regressions).

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
Cargo.toml	Adds proptest dev-dependency and a dedicated proptest integration test target.
tests/proptest/mod.rs	Wires the proptest integration test modules and documents harness behavior.
tests/proptest/strategies.rs	Defines proptest strategies for sides, makers, and books at a single level price.
tests/proptest/properties.rs	Implements the nine invariants as proptest properties over PriceLevel::match_order and snapshots.
tests/proptest/properties.proptest-regressions	Checks in regression seeds for CI replay of previously-found counterexamples.

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[joaquinbejar]

Thanks, all four addressed: the FIFO partial-fill check is now reachable and real (Standard front maker + monotonic book timestamps so snapshot/sweep ordering coincide), the regression-file doc path is corrected, the stale regressions file removed, and the book_strategy doc no longer overclaims non-empty.