Restore weakened reducer assertions and add maker runtime tests (fixes #222)

[wjllance]

Summary

Issue #222 flagged two gaps: commit ed849a2 (moving the reducer into core) silently weakened the reducer's test assertions, and the ~1500-line run_maker event loop has zero test coverage. This PR is test-only — no production behavior changes.

Restored the four assertions weakened by ed849a2 in crates/standx-maker/src/runtime.rs:

• critical_events_abort_stale_work_and_clean_up now asserts the phase enters Frozen.
• recovery_reconnects_only_after_cleanup now asserts the phase returns to Ready after RecoverySucceeded.
• unmatched_response_overflow_fails_closed now asserts benign pushes under the threshold return empty and that a Cleanup effect is emitted on overflow.
• coalesced_timer_replans_after_current_work re-adds the deleted assertion that MarketChanged merges (returns empty) while work is in flight.

Coverage added

New #[cfg(test)] module in crates/standx-cli/src/commands/maker/runtime.rs exercising the testable seams of run_maker:

• apply_order_response: accepted placement stays pending, rejected placement is removed, unmatched / missing request_id reported as unmatched.
• apply_order_responses: benign matched acks do not grow the unmatched buffer (no fail-closed freeze); a later matched ack clears a prior unmatched buffer entry.
• apply_account_events: position mismatch recorded with correct sign (buy positive, sell negative); buffered events applied in FIFO order (last position wins); benign Connected/TradeShadow drained without fills; updates for other symbols ignored.

Still uncovered

The run_maker async event loop itself (select over timers, market feed, account/order-response streams, cycle execution, cleanup/reconnect orchestration) is not driven end-to-end here — that would require a large mock-driven integration harness and architectural change, which #222 explicitly scoped out. Coverage was added by testing the already-extracted pure/synchronous seams rather than refactoring the loop. No new helper extraction was needed. Remaining uncovered surface: the top-level select loop, snapshot/cycle pipeline wiring, and stream reconnect/backfill glue.

Test plan

• cargo fmt -- --check — clean
• cargo clippy --workspace --all-targets -- -D warnings — clean
• cargo test -p standx-maker — 54 passing (4 restored reducer tests included)
• cargo test -p standx-cli — passing (8 new runtime seam tests included)

Fixes #222

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