Tweens: document what exists, and stop hand-rolling it

[obiot]

The engine has eleven easing families. The skills showed exactly one of
them, Quadratic.Out, twice. The one place that offered "the full list"
pointed at {@link Easing} — a page typedoc does not generate — so it went
nowhere, and nothing named Back, which is the difference between a fade and
an impact.

This started because I hand-rolled an easing curve by hand while the engine had
the named one sitting there, then found the same thing in two other examples.

Engine

• Tween.Easing carries the list itself: all eleven families, what each is
  for, and the trap that Back and Elastic leave the range between your
  endpoints, so an opacity or a colour channel has to be clamped. Plus the note
  that overshoot follows the direction of travel, which is not obvious: a value
  tweened DOWN overshoots BELOW its target.
• EasingFunction and InterpolationFunction exported as types. They were
  referenced by the documentation but not included in it — typedoc warned on
  both. Warnings go 166 → 164 and both type pages now exist.

easing.ts itself is untouched: Back has always been there. The gap was that
nothing pointed at it.

Skill

melonjs-scenes-and-state already had a good ## Tweens section. What it
lacked was the half that prevents the mistake rather than describing the
cure:

• "You are hand-rolling a tween if you write this" — the longhand shape
  beside the tween, and the tells: a *Ms/*Age field only update() touches;
  a division by a *_MS constant; k * k or ** 1.4 inline, which is an
  easing spelled by hand; an if (x <= 0) arm that only cleans up at the end,
  which is onComplete; a second timer counting the same duration, which is
  delay.
• "When NOT to reach for one" — a damp toward a moving target, an endless
  oscillation, a cooldown gate and a per-instance effect all look similar and
  are none of them tweens. The rule: fixed duration, known endpoint,
  one-shot, few instances.
• "Pick the easing, not just the duration" — Back and Bounce by name.

Its triggers now match what someone writing one would say (fade out,
flash, pulse, punch, countdown in update) rather than the word "Tween",
which is precisely what they would never search for. melonjs-ui-and-text had
no ## Related skills block at all; it has one now, and
melonjs-renderables / melonjs-effects-and-shaders gained the pointer.

Also: where a Text gradient actually lives

Same file, same class of problem, and the one that started the whole thread.

Text.fillStyle accepts a Gradient; it never holds one. The
constructor routes a gradient to fillGradient and leaves fillStyle as the
pooled Color it always is, which is what keeps fillStyle.alpha gating the
fill and keeps the colour owned by the pool. So changing a ramp after
construction means assigning the field it landed in:

label.fillGradient = ramp;   // ✓
label.fillStyle = ramp;      // ✗ TS2740: Gradient is not a Color

I hit that TS2740 and misread it as a typing gap, nearly "fixing" it by
widening fillStyle to Color | Gradient — which would have broken the
alpha > 0 fill gate, the copy() path and the pooling. The engine is right;
the documentation was missing. No typing changed here, only the skill.

The melonjs-ui-and-text skill also gains a table of what fillStyle means in
each class, since the name is shared and the behaviour is not: a real Color
on Text against Renderable#tint on BitmapText, where white is the
absence of a tint rather than white glyphs, gradients are impossible, there
is no stroke, and size is a ratio rather than pixels.

Every claim in that section was checked against text.js and bitmaptext.js
rather than written from memory.

Conversions

Two, both one-shot singletons with a curve written by hand:

• afterBurner's death wash. The effect lives on the camera rather than on a
  renderable, so nothing ticks it — which had forced a whole HUD#update(dt)
  and a call site in the game loop purely to drive the fade. Both are now
  gone. This is the case where a tween is not merely tidier: it removes
  machinery that existed only to work around not having one.
• jungleRabbit's multiplier punch, which was Quadratic.Out written out as
  1 + MULT_PUNCH * (1 - k) * (1 - k).

Deliberately not converted: per-enemy hit flashes, score popups and
contrail nodes. Dozens are live at once and a float subtract beats a tween
each — which is the "when not to" rule earning its place.

Also in afterBurner

GAME OVER was a fixed 42px. It is now sized to 82% of the viewport, measured
from a throwaway bake rather than scaled with a transform, because Text
rasterizes at its font size and a scaled-up bake would be blurry at rest.
It takes a dark outline at 3% of the size, since red text over the red death
wash had almost no contrast at the one moment you are meant to read it. And it
drops with the same Back.Out stamp used elsewhere — with no shake, since
the death already fires shake(22, 900, force).

Verification

Engine suite 7534 passed | 10 skipped, npm run doc 0 errors, biome clean
across 731 files. The examples package sits at its 564 pre-existing type
errors, unchanged (counted with ANSI codes stripped — grep "error TS" silently
matches nothing against coloured tsc output, which had been hiding the real
figure).

All three touched examples were loaded in a browser and driven through start,
input and restart with zero page or console errors.

🤖 Generated with Claude Code

https://claude.ai/code/session_01NGvtaUNATVCVxD2qcbiY4t

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