Cuckoo: quarter-hour nod + hourly trust flourish on the time row (opt-in)

[peterlewis]

A cuckoo for the Mk IV: short, scheduled display flourishes on the time row, after the Westminster
pattern — each quarter-hour gets a fixed small gesture (the nod: a shallow brightness trough
sweeping the six big digits left to right, ~550 ms, sub-seconds and colons untouched), and the hour
performs the configured piece.

Videos

Cuckoo (hour): https://www.youtube.com/watch?v=0jkTwYmk7kM
Cuckoo (quarter): Coming soon!

One config key, enable-and-select fused, off by default:

cuckoo = off        # off (default) | trust

trust — how much the instrument actually knows right now: the time row dips to a dim x-ray,
then a relight wave walks HH → MM → SS → ds → cs → ms, each digit relighting to the confidence the
tolerance ladder genuinely holds for it. Fully locked earns one unified pulse; in holdover the wave
dies at the honest position and the dashes return. The final frame of every flourish is the plain
live face — the clock never shows wrong digits.

How it renders. Pieces write per-digit intent levels (0..16, meaning perceived lightness);
the seg_balance dither mirror composes them into duty through a perceptual gamma
(16·(L/16)^2.2) — light output is linear in lit cycles, the eye is not, and the curve is what
made the choreography read on real LEDs. One interleave engine, two clients. With cuckoo = off
the display path is byte-identical to stock. Every envelope is a terminating integer countdown;
MODE_STANDBY and text/countdown modes refuse and abort.

Preview. The on-device menu row (DISP → CUCKOO) performs each value as it is selected — the
menu's colon-preview idiom applied to the flourishes — and cuckoo_preview = trust|nod|on over USB
serial plays once without touching the config. The selection persists in the on-device settings
store (new key id in previously-zeroed record padding; old records round-trip, schema unchanged).

Scope honesty. Time-board-only, for two hardware reasons: the time row reads left-to-right in
both fold poses (only the date board rotates, and it senses its own orientation), and the date
row's UART carries text with no brightness — so whatever it shows (date, ADEV, star transit,
countdown, text) keeps reading truthfully through every flourish. CUCKOO_SPEC.md (in this PR)
records the full design: the cadence law, the stand-in rule, and four further pieces (carry,
heartbeat, pendulum, rain) specified but not shipped — three parked pending visual rework, rain
deferred to the date board; their names parse to off.

Verified: WASM conformance harness 23/23 (envelopes and termination on the plain face,
quarter/hour scheduling through real boundaries, off byte-identity, parked-name safety, preview
semantics); settings-store and factory-reset suites pass; ARM Release clean (174 756 B text);
running on a bench Mk IV. Draft note: the nod's depth (8/16 floor) and sweep rate (30 ms/digit)
are bench-tuned once and marked provisional in the spec.

Stack: based on menu (#13) — followed by zone2 (#15). Merging #5 → … → #13 → this PR
→ #15 in order is a chain of fast-forwards whose result is byte-identical to the
rollup branch (the tree this feature was
emulator-tested and hardware-validated on).

🤖 Generated with Claude Code