feat: add clock_ticker for apps that only need the tick count

[ArthurGibert]

Reopens #644, which was merged by mistake on 2026-08-14 and then reverted from main (force-push reset to the pre-merge commit). Same content, unchanged: three stacked commits (fix/clock-gatekeeper, feat/clock-ticker, feat/app-clock-ticker), since TICK_COUNTER is introduced further down the stack.

See #644 for the original description and review discussion.

[chmanie]

The ticks are now part of the clock events that's why it was removed earlier. See here:

faderpunk/faderpunk/src/app.rs

Line 317 in 25546b5

ClockEvent::Tick(ticks) => {

[ArthurGibert]

I guess as a work-around one could still subscribe to clock events and use a global variable to share the count to the other loops. Now we just need to decide if that's how we want this case to be handled @chmanie .

For context PR #644 was meant to bring back the ticker for apps that do not use the clock events:

App::clock_ticker() hands an app a plain fn() -> u64 that reads the current 24 PPQN tick out of TICK_COUNTER. Apps that derive everything from the tick number and never react to Start/Stop/Reset can use it instead of use_clock(): it costs no CLOCK_PUBSUB subscriber slot, and an app that falls behind cannot back up the shared clock queue. u64::MAX means “not started”; the counter going backwards means the clock was restarted or reset.

It is #[allow(dead_code)] on main — the first users are the WIP app branches, where a clocked app that also awaits a MAX write or a MIDI send inside its loop is exactly the subscriber that used to stall the gatekeeper.

Stacked on https://github.com/ATOVproject/faderpunk/pull/638 (which is itself stacked on https://github.com/ATOVproject/faderpunk/pull/636), because TICK_COUNTER is introduced there. Cross-repo PRs can only target main, so this diff shows all three commits; merge https://github.com/ATOVproject/faderpunk/pull/636 and https://github.com/ATOVproject/faderpunk/pull/638 first and it shrinks to the app.rs change alone.

No test checklist: this adds an unused accessor and changes no behaviour on its own.

Opened by an AI coding agent on @kosmar's behalf.```