perf(allocate): skip the no-op header CAS for unfillable backorders (#67)

[JumpTechCode]

Closes #67.

Problem

When the sweep re-allocates a backordered order and no stock is available, allocate computed target = BACKORDERED == order.state and then called cas_state unconditionally, executing UPDATE orders SET state='backordered', version=version+1 — a real row write, dead tuple, WAL record, and version bump on every tick even though nothing changed. With B orders backordered on a zero-stock SKU (a common steady state), each order incurred one header UPDATE per interval indefinitely: WAL/IO and autovacuum pressure proportional to backlog × tick rate, dead tuples on the hot orders table, and a modestly raised OCC-conflict probability for legitimate concurrent allocate/cancel.

Fix

Track whether any line gained allocation this pass (any_allocated) and skip cas_state entirely when the state does not change and nothing was allocated. A real state change or any new line allocation still CASes — both to persist the transition and to keep the version bump that serializes concurrent allocate/cancel against the lines this pass just changed. Steady-state cost on a perpetually-unfillable order drops from O(backlog × tick) to ~0.

No invariant is affected: in the zero-available case no stock is touched (stock_locations filters available > 0, so the per-line FOR UPDATE reserve never runs).

Scope

This addresses the write-amplification core of #67. The optional read-side filter (excluding zero-available SKUs from backordered_orders) and per-order backoff are deliberately left out to keep the change minimal; the no-op-write elimination already removes the dominant steady-state cost.

Tests

• Unit (test_allocate_handler.py): a no-progress backordered re-allocation now asserts no cas_state call; new guard test that partial progress on a backordered order still CASes; strengthened the CREATED→BACKORDERED test to assert the first-time transition still persists.
• Integration (test_backorder_sweep.py): test_zero_stock_sweep_does_not_rewrite_the_order runs the sweep 3× against a zero-stock backordered order and asserts the header version is unchanged across ticks.

make verify green locally: 505 passed, 99.14% coverage.

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