fix(EffectComposer): fix StrictMode composer leak, guard against r3f reorder bug

[kvvasuu]

Stacked PR 3/6 — part of a split of #350 .
← based on #352 · → followed by #354

Note: dispose coverage for ColorAverage isn't added here on purpose —
ColorAverage doesn't dispose itself until #355.

Fixes #270

EffectComposerImpl was created inside useMemo, which doesn't guarantee its cleanup runs before React discards a memoized value - this leaked the composer (and its WebGL resources) under StrictMode. It's now created and disposed inside a useState/useEffect lifecycle instead.

Pass collection walks the real r3f instance tree (group.current.__r3f .children) and rebuilds the pass list from scratch whenever it or the composer changes, so the order always matches current JSX - including through wrapper components - after a reorder or a remount.

While testing the reorder case, found a genuine bug in r3f's host config: when React moves (not remounts) an existing child - e.g. a key-preserving reorder of a keyed effect list - insertBefore/ appendChild splice the moved instance into its new slot without detaching it from the old one first, leaving a stale duplicate in Instance.children. Worth reporting upstream. Added a cheap dedupe (keep each object's last occurrence, sorted by that position) that recovers the correct order either way and is a no-op when the bug isn't present.

Also widens children from JSX.Element | JSX.Element[] to ReactNode, which was rejecting the common condition && <Effect/> idiom (that expression is false | Element, and false isn't assignable to either half of the union). The tree-walk already tolerates arbitrary children gracefully - it just filters for instanceof Effect || instanceof Pass - so the stricter type wasn't buying any actual safety.

Replaces the old root-level EffectComposer.test.tsx (superseded, predates this suite and used a different mock-root setup) with a suite covering pass registration/composition order, StrictMode disposal, and the reorder/dedupe behavior above. (ColorAverage-specific dispose coverage lands with the primitive-effect dispose fix, a few commits later - ColorAverage doesn't dispose itself until then.)