feat(textures) Add composite texture loaders

[ibgreen]

Summary

Add manifest-driven composite image texture loaders to @loaders.gl/textures, deprecate the legacy helper APIs, and standardize composite-loader base URL handling around LoaderContext.

This PR introduces first-class loaders for image textures, texture arrays, cubemaps, and cube arrays, all backed by a shared composite-image parser/resolver. It also adds template-based mip expansion, aligns cubemap face naming with luma.gl conventions, and documents how composite loaders should forward base URLs and subloader lists.

Included Changes

• Added new public loaders in @loaders.gl/textures:

  • ImageTextureLoader
  • ImageTextureArrayLoader
  • ImageTextureCubeLoader
  • ImageTextureCubeArrayLoader

• Added JSON manifest support for:

  • single images
  • mip chains
  • texture arrays
  • cubemaps
  • cube arrays

• Added template-based source expansion with strict validation:

  • supports mipLevels: "auto"
  • supports placeholders such as {lod}, {index}, {face}, {direction}, {axis}, {sign}
  • supports escaped braces via \{ and \}

• Aligned cubemap manifest keys with luma.gl conventions:

  • canonical keys are +X, -X, +Y, -Y, +Z, -Z
  • legacy directional aliases like right, left, top, bottom, front, back are still accepted for compatibility

• Reimplemented legacy loadImageTexture* helpers on top of the shared composite-image path and marked them @deprecated

• Standardized relative URL resolution:

  • prefer context.baseUrl
  • use options.core.baseUrl only as a pre-context fallback
  • stop forwarding base URL state ad hoc between composite loaders

• Updated composite member parsing so subresources use:

  • top-level fetch
  • top-level loader arrays in addition to ImageLoader
  • child context with the correct member url / baseUrl

Documentation

• Added API docs for the new texture manifest loaders
• Moved composite loader base URL guidance into the general composite loader developer guide
• Removed legacy helper guidance from regular docs
• Kept helper migration guidance in the upgrade guide only

Tests

Added coverage for:

• manifest loading for image, array, cube, and cube-array loaders
• relative URL resolution
• core.baseUrl fallback for in-memory manifests
• template-based mip expansion
• escaped brace handling
• template validation errors
• top-level fetch reuse for member assets
• top-level loader forwarding for composite members
• loader auto-selection by manifest shape

Verification

• ./node_modules/.bin/tsc -p modules/textures/tsconfig.json --noEmit

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