feat: runtime waveform override api

[markomitranic]

Why

let apps tune the grayscale look live from app space — a
settings panel with a −/+ "brightness" slider over the mid grays, plus a grayscale on/off
toggle. Today the LUT is baked into Settings at compile time, so calibrating a new panel
means editing the library and reflashing for every tweak. This adds a runtime channel: an
app pushes its own init/LUT registers and sees the result in ~1s, no rebuild.

It also retires the ad-hoc gc4_tune calibration scaffolding — the way you calibrate the
next panel is now just set_waveform + a normal draw.

* The test photo is Kodak "kodim17" from the [Kodak Lossless True Color Image Suite](https://r0k.us/graphics/kodak/)

What changed

Two layers plus a semantic builder.

1. A generic override channel on EInk — the public API:

  set_waveform(:grayscale, lut: […])
        │  stores the override in GenServer state, keyed by mode
        │  calls driver.reset  ──▶  active_state: nil   (chip forgets its mode)
        ▼
  next EInk.draw(img, mode: :grayscale)
        │  GenServer merges the mode's override into the draw opts
        ▼  driver re-inits and uses opts[:lut] instead of Settings.get_lut

• EInk.set_waveform(mode, init: […], lut: […]) — both keys optional, stored per mode.
• EInk.clear_waveform(mode) — drops it, back to the packaged defaults.
• Cost: set/clear force one full re-init (~1s) on the next draw. By design — the LUT lives
  in chip registers that only a fresh init reloads reliably.

2. Driver plumbing. All three drivers (uc8276, uc8179, uc8253c) now prefer
opts[:init] / opts[:lut] over Settings.get_init / get_lut everywhere Settings is
consulted — including uc8179's post-draw data-interval read and uc8253c's active_lut_reg
ping-pong. Nothing else in the drivers changed.

3. A semantic builder UC8276.Settings.grayscale_lut([black, dark, light, white], reset)
— builds the five LUT registers from four whiten-frame counts instead of five opaque byte
literals. get_lut(:grayscale, _) now calls grayscale_lut([0, 5, 10, 54], 63), verified
byte-identical to the previous hand-tuned literals, so on-device behavior is unchanged.

The slider flow it enables

  slider k  ──▶  set_waveform(:grayscale, lut: grayscale_lut([0, 5+k, 10+k, 54]))
                 keep black/white anchors fixed, nudge only the mids

eink stays deliberately dumb here: it exposes the raw frame counts and nothing else. The
mapping from a friendly slider to counts — and clamping to the panel's usable frame window —
is the app's policy, not the library's.

Stacking

This sits on top of #11 (the grayscale LUT data). Until that merges, the diff below includes
#11's commit (add 4-level grayscale init and LUT…). The actual new work here is the single
add runtime waveform-override api commit.