enable noUncheckedIndexedAccess and resolve the resulting errors

[vpmedia]

What. noUncheckedIndexedAccess is the one compiler flag @vpmedia/phaser
still lacks that its siblings @vpmedia/bitcharify and @vpmedia/simplify
already hold. Enabling it reports 177 errors.

Why. Every remaining unchecked index read is a place where an out-of-range
lookup yields undefined and the code carries on with it. The rest of the
strictness work landed in 8efcb9a, 8f747a1 and 6dcfb35; this is the last
gap.

Why it was deferred. The 177 sites concentrate in the render, particle,
tween and earcut inner loops, where a per-element undefined guard is both a
per-frame runtime cost and noise in the hottest code in the library. That
trade-off deserves its own review rather than riding along with a release that
already changes the public nullability surface.

Prompt:

Enable noUncheckedIndexedAccess in tsconfig.json and resolve the 177 errors.

- Work file by file: `npx tsc --noEmit --noUncheckedIndexedAccess`.
- Prefer iterating the collection directly (`for...of`, `map`) over indexing;
  that removes the error rather than suppressing it, and reads better.
- Where the index is provably in range and the loop is per-frame hot, hoist the
  element into a local and guard once, rather than re-reading the index.
- Do not reach for `as` or `!` to silence a read. If a hot path genuinely cannot
  afford the check, say so in the PR and leave that file for a follow-up rather
  than asserting past it.
- Acceptance: `pnpm typecheck`, `pnpm lint`, `pnpm test` and `pnpm build` clean;
  bundle size not materially larger than the 646 kB the flag-free build produces.