feat: Loop de Cay additive overdub loop

[kosmar]

Summary

• New 1ch app Loop de Cay: clocked additive overdub loop with per-wrap level decay (kill-floor to true 0)
• Five I/O modes via Config: Pitch→MIDI, Gate→MIDI, MIDI→MIDI, MIDI→CV, Fader→Both
• Gestures: Press = gate/play, Long = mute (no fader move), Shift+Short = arm, Shift+Long = erase
• Alt = decay, Third = 1–8 bar window (crop/reveal + virgin tile); poly 4; RAM-only buffer
• App ID 38 (avoids Hold Sam 36 / Harmonica 37 WIP IDs)

Test plan

• Flash firmware and add Loop de Cay on a channel
• Fader→Both: arm, play short notes with button+fader, confirm loop repeats on clock
• Overdub adds notes; Alt decay fades velocity to silence; mute pauses decay
• Shift+Long erases; Third shortens/extends bars (tile on virgin extend, reveal after crop)
• Spot-check other modes (Pitch→MIDI, Gate→MIDI, MIDI→MIDI, MIDI→CV)

Prerequisites

The platform pieces this app needs are under review as their own PRs, so that
each app PR stays small enough to review on its own.

Required — these APIs do not exist on current main:

• fix: keep the metronome off the clock pubsub #638 — tick counter plus App::clock_ticker, which replaces the Clock::get_ticker this branch was written against
• feat: let apps write back their own params and notify the host #640 — ParamStore::update, so an app can write its own params back to the host

This branch is still based on an older main that still had the removed clock
helper, which is why it builds on its own but not after a rebase.

On top of the PRs above there is an app-side change: ClockEvent::Tick became
Tick(u64) in #579, so the match arm here has to be updated.

Building any of this on a current Rust nightly also needs the toolchain fix in
#632.

[kosmar]

Update: Pitch→MIDI fader + Span

e9822be — In Pitch→MIDI, Mode 0 always opens a CV in jack and resolve_pitch preferred that path, so the fader (and Span) never drove pitch when unpatched. Open CV rests near 0 V and pinned notes to Base Note; Hold+Fader glissando was dead for the same reason.

CV pitch and the bipolar fader offset now stack: unpatched the fader keeps the documented span; with a cable it transposes; centre = 0 semitones (also the saved default).

Test on hardware

• Loop de Cay · Pitch→MIDI · Base Note C3 · Span 120 · no cable on the jack
• Hold button + move fader → full ±60 semitone range (scale-quantized), not stuck on C3
• Patch V/Oct: centre = unchanged CV pitch; move fader to transpose

[kosmar]

Pushed two fixes found while testing on the playground build:

• fix(loop-de-cay): let a plain hold sustain again — the Hold+Fader glissando chased its target unconditionally, and since the target includes CV in, an unpatched jack jittering across a quantizer step retriggered the held note. A plain hold now sustains; the glide only engages once the fader has actually moved during the hold.
• fix(loop-de-cay): end loop voices even when clock ticks are skipped — note starts and ends tested the loop position for exact equality. The clock gatekeeper publishes ticks immediately, so a subscriber that falls behind drops them rather than stalling the device clock, and a missed end position left the note sounding until its voice slot was stolen. Both now test whether the position was crossed since the last tick, wrap-around included.

Written and pushed by an AI coding agent on behalf of @kosmar. Verified to build; hardware test still pending.