fix(precompute): force-close open windows on worker shutdown

[zzylol]

Problem

flush_all only advances the per-group watermark by +1ms (plus the wall-clock fallback, which is gated on grace having elapsed on a flush tick). For a one-shot batch ingest whose records all fall in a single window — so event-time never advances past the window end — the trailing window never closes, emit_batch is never called, and that window's data never reaches the store.

The Shutdown handler previously just called flush_all(), so the trailing window was lost.

Fix

Add Worker::force_close_all(), invoked from the Shutdown handler after the final flush_all(). For every bucket with open panes it closes and emits every remaining window unconditionally (no more samples will arrive after shutdown), covering both pane maps:

• active_panes (sample aggregation) → merge_panes_for_window
• sketch_panes (OTLP-delivered sketches) → merge_sketch_panes_for_window

To advance past the open windows it uses a finite bound derived from the largest open pane (max_pane + window_size_ms), not i64::MAX: WindowManager::closed_windows enumerates window starts one slide at a time, so i64::MAX would loop ~i64::MAX/slide times and overflow. max_pane + window_size_ms is the smallest watermark that closes the latest open window.

Idempotent: drained panes are removed from both maps and their wall-clock bookkeeping is pruned, so a second call emits nothing.

Tests

• shutdown_force_close_emits_trailing_sample_window (active_panes path)
• shutdown_force_close_emits_trailing_sketch_window (sketch_panes path, 10 DDSketches at frozen event-time → one merged window emitted with total_count() == 10)

Verification

• cargo check --lib --tests — clean (pre-existing warnings only)
• precompute_engine::worker — 30 passed (incl. 2 new)
• precompute_engine (module) — 241 passed

Context

Companion to ProjectASAP/ASAPQuery#400, which ports the wall-clock grace fallback into the frontend and adds the same shutdown force-close there. This PR adds the shutdown force-close to the backend, where the wall-clock fallback already exists.

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