Release 0.8.2: expose matchable_quantity + add snapshot_by_seq_into (buffer reuse)

[joaquinbejar]

Closes #104.

Two small, additive, non-breaking API changes, shipped as 0.8.2. Neither changes existing behavior, dependencies, matching semantics, or the snapshot wire format.

Changes

1. PriceLevel::matchable_quantity is now pub

It was already the deterministic fill-or-kill dry run — a no-mutation replay of the FIFO sweep (including iceberg / reserve replenishment) that returns exactly what match_order would consume. Making it public lets a composing order book delegate per-level all-or-nothing feasibility to this single upstream source of truth instead of re-implementing the sweep and risking drift. Now #[must_use], logic unchanged.

2. snapshot_by_seq_into(&self, out: &mut Vec<Arc<OrderType<()>>>) (buffer reuse)

A buffer-reuse variant of snapshot_by_insertion_seq(): clears out and refills it in ascending insertion sequence (the order match_order consumes orders), so a consumer that walks every level repeatedly (e.g. a self-trade-prevention pre-scan) can reuse one pooled scratch buffer and avoid the per-call allocation. Added on both PriceLevel (pub) and OrderQueue (pub(crate)); the existing snapshot_by_seq() now delegates to the _into form (single walk definition).

Housekeeping

• Bump Cargo.toml 0.8.1 → 0.8.2.
• New CHANGELOG.md (Keep a Changelog style) with the 0.8.2 entry plus back-fill for 0.8.1 / 0.8.0.

Tests (src/price_level/tests/level.rs)

• test_snapshot_by_seq_into_matches_snapshot_by_insertion_seq — same sequence as the owned-Vec variant.
• test_snapshot_by_seq_into_reuses_buffer — seeds the buffer non-empty, then reuses it across a smaller and a larger level; asserts clear() (no stale tail) and correct grow/shrink contents.
• test_matchable_quantity_public_predicts_sweep — the now-public method equals what match_order actually consumes (depth cap + iceberg replenishment parity).

Verification

• make pre-push clean: 415 lib tests (412 + 3 new), 9 proptests, 1 integration, 9 doctests; clippy + fmt + rustdoc -D warnings all green.
• No README.md diff (item docs only).

No new exported type → no lib.rs / prelude.rs re-export change and no migration-guide entry.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/price_level/level.rs	93.12% <100.00%> (+0.30%)	⬆️
src/price_level/order_queue.rs	93.75% <100.00%> (+0.25%)	⬆️

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