fix(examples): replace textureOptions wholesale instead of mutating frozen default

[chiefcll]

Problem

The default examples page (?test=test) throws repeatedly on load:

TypeError: Cannot add property flipX, object is not extensible
    at nextFrame (examples/common/Character.ts:60)

Nodes created without textureOptions resolve to a single shared Object.freeze({}) default (EMPTY_TEXTURE_OPTIONS in src/core/Stage.ts), whose documented contract is "only ever read or replaced wholesale (never mutated in place)". Two examples violated that contract by mutating the object in place:

• examples/common/Character.ts — node.textureOptions.flipX = flipX on every animation frame
• examples/tests/texture-cleanup-critical.ts — screen.textureOptions.preload = true on every interval tick (same latent bug, different test page)

Fix

Assign a new options object through the textureOptions setter instead of mutating:

• Character: this.node.textureOptions = { flipX } once per direction change. Safe because the texture setter resets textureCoords on every frame swap and coords are recomputed from the current options, so the flip applies to each new frame.
• texture-cleanup-critical: screen.textureOptions = { preload: true } once before the interval; preload is read on each texture load.

Verification

Loaded http://localhost:5173/?test=test with the console open: zero errors after the fix (confirmed against a console marker to rule out stale log entries), and screenshots ~30s apart show the Rocko sprites still animating (walk frames advancing). Prettier + ESLint pass.

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