DST harness + README alignment for frozen-snapshot periods

[pbudzik]

Summary

• Adds tests/dst.rs — a Deterministic Simulation Testing harness that drives random sequences of Ingest / Flush / RollupTick / CompactTick / Restart / ClosePeriod against a parallel reference model. After every step it asserts §19.5 (raw sum per account matches the model); at the end of each sequence it does a final flush + rebuild_rollups + tick + compact and asserts §19.8 (raw == rollup). 64 cases per run; failures shrink to a minimum-length op trace.
• Aligns README with the closed-period frozen-snapshot semantics from PR Frozen-snapshot semantics for closed periods #23 — the GET /periods/{YYYY-MM} endpoint now returns frozen + pending_adjustments + net_total for closed periods, not just state + total. Adds a Period lifecycle subsection and a Status bullet for the DST harness.

Notes

• The original failing seed (a RollupTick on an empty DB, advancing the watermark past hour 0 before any events arrived) is checked in via tests/dst.proptest-regressions so CI replays it before generating novel cases. The fix in the harness is to call rebuild_rollups(0, i64::MAX) before the final reconciliation tick — same primitive operators use in production when rollups drift.
• The reference model deliberately stays minimal: acked-event hashes, per-account totals, closed period set. It does not model the rollup watermark or segment layout — the SUT's raw query reflects all of that, which is exactly what we want to compare against.

Test plan

• cargo test — all 145 tests pass (139 existing + 1 DST + new properties)
• cargo build --release clean
• DST seed for the originally-found watermark-advance edge case is captured in the regressions file

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