feat: Hold Sam clocked CV sample-and-hold to MIDI

[kosmar]

Summary

• New 1-channel app Hold Sam (ID 36): clocked sample-and-hold from CV in to MIDI Note/CC
• Note mode: cut+retrigger each division tick; gate % from Main; velocity = base + sensitivity × |ΔCV|
• Soft mute / Stop ausklingen; long-press without fader move = panic; Shift+short = manual sample
• Clippy fix: as_chunks::<4>() in USB MIDI RX path

Test plan

• Flash firmware and add Hold Sam on one channel
• Patch CV into the jack; confirm Note mode fires on global clock with gate length from Main
• Confirm Alt sets division and Third changes velocity with CV jumps
• Soft mute: note finishes gate; long-press (no fader move): hard Note Off + muted
• Shift+short manual sample works; CC mode sends scaled CC
• Stop leaves note to finish; Reset keeps sample/note

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

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.