test(ledger): add proptest coverage for balance math

[nicolasburtey]

Summary

Adds property-based test coverage for the pure balance math in `cala-ledger` — the closest CALA analog to "debits == credits" invariants. 11 properties at 4096 cases each, ~0.7s combined runtime. No production code changes.

`Snapshots` (cala-ledger/src/balance/snapshot.rs) — 5 properties

• `new_snapshot_only_touches_target_cell` — initial snapshot has all-zero balance cells except the (layer, direction) the entry targets.
• `version_increments_by_one` — every `update_snapshot` bumps version by 1.
• `sum_of_units_lands_on_target_cell` — N entries to one (layer, direction) yield total == sum of units; cross-layer cells stay zero.
• `balance_amounts_are_order_independent` — applying any permutation of the same multiset of entries yields the same balance amounts. The additive-commutativity property the per-account double-entry contract rests on.
• `available_rollup_composition` — `BalanceSnapshot::available` rollup is (settled, settled+pending, settled+pending+encumbrance) on dr and cr.

`AccountBalance` / `BalanceWithDirection` (cala-ledger/src/balance/account_balance.rs) — 6 properties

• `settled_direction_dependent` — Credit account: `cr - dr`; Debit account: `dr - cr`; sign flip on direction.
• `pending_and_encumbrance_direction_dependent` — same flip rule.
• `available_layered_composition` — `available(Settled) == settled()`; `available(Pending) == pending() + settled()`; `available(Encumbrance) == encumbrance() + pending() + settled()`.
• `account_balance_available_matches_view` — `AccountBalance::available` agrees with `BalanceWithDirection::available` for any direction/layer.
• `derive_diff_with_zero_is_identity` — diff against zero baseline returns self on all four amount fields.
• `derive_diff_with_self_is_zero` — diff against self returns zero on all four amount fields.

Notes

• Tests live as inline `#[cfg(test)] mod proptests` next to the implementations, since `Snapshots::new_snapshot` and `update_snapshot` are `pub(crate)`.
• `proptest = "1.5"` added to workspace dependencies and to `cala-ledger` `[dev-dependencies]`.
• All entries in the test fixtures use `Uuid::nil()` for ids since the balance math under test only reads `entry.layer`, `entry.direction`, and `entry.units` — id values don't affect the asserted invariants.
• Existing 22 unit tests in `balance::` continue to pass.

Test plan

• `cargo test -p cala-ledger --lib balance::` — 33/33 pass (11 new + 22 existing)
• CI green

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Note

Low Risk
Adds only property-based tests and dev-dependencies (proptest) around balance/snapshot math, with no production logic changes. Main risk is increased test runtime/flakiness or dependency/lockfile churn.

Overview
Adds proptest to the workspace and cala-ledger dev-dependencies (updating Cargo.lock) to enable property-based testing.

Introduces new #[cfg(test)] proptest modules in balance/account_balance.rs and balance/snapshot.rs that exercise invariants for direction-aware signed balances, available() rollups, snapshot update behavior (cell targeting, version increments, sum-of-units), order-independence, and AccountBalance::derive_diff identity/zero-diff properties.

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[github-actions]

📊 Performance Report

Commit: be424d5
Updated: 2026-05-10 12:21:41 UTC

Cala Performance Benchmark Results (non-representative)

Criterion Benchmark Results (single-threaded)

Benchmark	Time per Run	Throughput	% vs Baseline
post_simple_transaction	7.151ms	139 tx/s	0 (baseline)
post_and_recalculate_ec_account_set	17.119ms	58 tx/s	-139.0%
post_and_batch_recalculate_ec_account_set	12.586ms	79 tx/s	-75.0%
post_multi_layer_transaction	8.404ms	118 tx/s	-17.0%
post_simple_transaction_with_effective_balances	8.606ms	116 tx/s	-20.0%
post_simple_transaction_with_skipped_velocity	6.373ms	156 tx/s	+10.0%
post_simple_transaction_with_velocity	7.829ms	127 tx/s	-9.0%
post_simple_transaction_with_hit_velocity	4.134ms	241 tx/s	+42.0%
post_simple_transaction_with_one_account_set	6.666ms	150 tx/s	+6.0%
post_simple_transaction_with_five_account_sets	7.994ms	125 tx/s	-11.0%
post_simple_transaction_with_ec_account_set	6.334ms	157 tx/s	+11.0%

Load Testing Results (parallel-execution)

Scenario	tx/s
1 parallel	83.45
2 parallel	137.41
5 parallel	157.60
10 parallel	161.01
20 parallel	159.36
2 contention	121.04
5 contention	139.22
2 acct_sets	111.16
5 acct_sets	123.37

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Note: Performance results may vary based on system resources and database state.

Last updated by commit be424d5