[codex] Add manual timer and shared queue core

[Bathtor]

What changed

• add a new manual_timer module that exposes the same queue-backed scheduling API as the threaded timer while letting tests advance logical time explicitly
• factor the shared queue-backed timer machinery into queue_timer.rs and reuse it from thread_timer.rs
• add TimerRef::now() support for both realtime and manual clocks
• document the new high-level API in the crate docs and README
• bump rustfmt.toml to required_version = "1.9.0"

Why

This ports the custom timer work from the flotsync-git patch into the crate itself so the manual timer support and shared TimerRef clock behaviour can be reviewed and maintained here.

Impact

• tests can drive timeout delivery deterministically without a worker thread
• threaded and manual timers now share the same queue-backed scheduling core instead of duplicating logic
• the now() API has explicit contract coverage for both manual and threaded handlers

Validation

• bash ./test-features.sh
• cargo test
• cargo test --no-default-features --features thread-timer,fnv-hash,uuid-extras
• cargo test --no-default-features --features thread-timer,sip-hash,uuid-extras
• cargo test thread_timer::tests::timer_ref_now_inside_handler_is_not_earlier_than_due_time
• cargo test --no-default-features --features thread-timer,fnv-hash,uuid-extras thread_timer::tests::timer_ref_now_inside_handler_is_not_earlier_than_due_time
• cargo test --no-default-features --features thread-timer,sip-hash,uuid-extras thread_timer::tests::timer_ref_now_inside_handler_is_not_earlier_than_due_time
• cargo fmt --all