Add schedule_timer_until absolute-deadline timer

[pinodeca]

Closes #34.

Summary

Adds OrchestrationContext::schedule_timer_until(deadline: std::time::SystemTime) — an absolute wall-clock deadline timer that complements the existing relative schedule_timer(delay).

/// Schedule a timer that fires at an absolute wall-clock deadline.
pub fn schedule_timer_until(&self, deadline: std::time::SystemTime) -> DurableFuture<()>;

This matches domains where the target is intrinsically an absolute timestamp (cron ticks, SLAs, scheduled-at times — e.g. pg_durable's wait_for_schedule), removing the manual deadline - now clamp and the extra recorded utc_now reading the relative form requires.

Implementation

• Converts deadline to ms-since-epoch and emits Action::CreateTimer { fire_at_ms } directly, reusing the existing timer future/replay path.
• A deadline already in the past (or pre-epoch) clamps to fire_at_ms = 0, firing immediately on the next turn.
• schedule_timer and schedule_timer_until now share a private schedule_timer_at_ms helper, keeping a single timer code path.

Backwards compatibility

Purely additive. No changes to existing signatures, the on-history action/event schema (Action::CreateTimer / TimerCreated already store an absolute fire_at_ms), the provider's visible_at handling, or replay semantics. Safe for rolling/mixed-version clusters.

Tests

• schedule_timer_until_fires_at_deadline — fires at an absolute deadline.
• schedule_timer_until_past_deadline_fires_immediately — past deadline fires immediately.

Validated with cargo nt, cargo clippy --all-targets --all-features (clean), and cargo test --doc.