feat(precompute): separate idle and deadline closure

[zzylol]

Before

The precompute worker used a single wall-clock grace concept, so an idle timeout and an absolute maximum-open deadline could not be configured or reasoned about independently.

After

• Track pane first-touch and last-touch wall-clock timestamps separately.
• Add distinct wall_clock_idle_grace_period_ms and wall_clock_max_open_grace_period_ms settings.
• Keep the old wall_clock_grace_period_ms spelling as a deserialize-only alias for idle grace.
• Close idle panes using last activity and bound continuously active panes using the absolute deadline.
• Add regressions for active ingestion and deadline closure.

Validation

• cargo fmt -p data_plane -- --check
• CARGO_TARGET_DIR=/mydata/ASAPQuery-backend/target cargo test -p data_plane precompute_engine:: (262 passed at this stack stage)

Supersedes #484, which GitHub automatically closed when its merged base branch was deleted.