feat(precompute): separate idle and deadline closure

[zzylol]

Why

An idle timeout and an absolute freshness deadline are different policies. PR #480 represented both with one last-touch timestamp, so continuously active fixed-timestamp input could remain open indefinitely.

Before this PR

A pane tracked only its latest wall-clock touch. The single wall_clock_grace_period_ms setting implemented idle closure and could not bound the lifetime of an active pane.

After this PR

• Track each pane first-touch and last-touch wall-clock times together.
• Rename the idle setting to wall_clock_idle_grace_period_ms; accept the old YAML name as a compatibility alias.
• Add the independent wall_clock_max_open_grace_period_ms absolute deadline.
• Keep the absolute deadline disabled by default until the follow-up guarantees correction forwarding.
• Advance only the closure watermark when either wall-clock policy fires.

For a one-second window with 5-second grace, idle closure fires six seconds after the latest touch, while the optional absolute deadline fires six seconds after the first touch even if the pane remains active.

Verification

• cargo fmt -p data_plane -- --check
• cargo test -p data_plane precompute_engine:: -- --nocapture (262 passed)

Includes regression coverage for the absolute deadline and legacy configuration deserialization.

Stacked on #483; retarget to main after #483 merges.