fix(precompute): preserve deadline corrections

[zzylol]

Before

An absolute wall-clock deadline could close an active event-time pane while the default late-data policy still dropped later inputs. That bounded freshness by silently losing data. There was also no metric separating dropped late inputs from appended corrections.

After

• Enable the absolute deadline only when ForwardToStore guarantees mergeable correction output.
• Make ForwardToStore the runtime/default policy and use a 5-second maximum-open grace.
• Preserve post-deadline raw Sum and prebuilt DDSketch inputs as mergeable corrections.
• Export asap_precompute_late_inputs_total{action,input_kind} for drop/correction observability.
• Document the corrected deadline and late-data contract.

Validation

• cargo fmt -p data_plane -- --check
• CARGO_TARGET_DIR=/mydata/ASAPQuery-backend/target cargo test -p data_plane precompute_engine:: (265 passed)
• Regression tests cover Sum correction merge, DDSketch correction merge, and disabling deadline closure under Drop.

Stacked on #484; merge after #483 and #484.