Fix sky vertical UV orientation and add per-sky flipV option

[tracygardner]

Motivation

• Sky textures and shader materials were using the same positive scaling for both U and V, but the vertical orientation for sky should be inverted while keeping uScale positive.
• Provide a way to opt out when inversion causes mirrored appearances by exposing a per-sky toggle rather than changing global material behavior.

Description

• In api/scene.js (flockScene.setSky) added an options.flipV parameter defaulting to true and kept clear behavior unchanged.
• Compute verticalScale = flipV ? -scaleValue : scaleValue and apply it only in the sky-material branch.
• For standard textures set tex.uScale = scaleValue and tex.vScale = verticalScale, and for shader materials call color.setFloat("uScale", scaleValue) and color.setFloat("vScale", verticalScale).
• No global material creation logic was modified; changes are scoped to sky setup only in setSky.

Testing

• node --check api/scene.js completed successfully.
• npm -s run test:api failed in this environment because Playwright browser binaries are not installed (error: missing Chromium executable).
• npm -s run lint reported pre-existing unrelated lint issues in flock.js and did not pass in this environment.

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Codex Task