feat: add ordered model library directories

[PhilipJohnBasile]

Adds ordered model library directories so one MTPLX installation can discover models across a primary cache and additional user-selected roots.

• keeps the existing cache as the first search root
• supports multiple ordered directories with deterministic discovery and deduplication
• carries the roots through model resolution, downloads, updates, diagnostics, retrieval, Forge, CLI, and the Swift settings and picker surfaces
• preserves safe path handling and existing cache behavior
• carries configured roots through interactive start and serve onboarding

Checks:

• full Python suite passed locally
• hosted no-MLX pytest subset: 842 passed locally
• changed-file Ruff and compileall passed
• Swift core tests: 660 executed, 1 skipped, 0 failures
• Swift host tests: 8 passed after placing the cached Sparkle framework in the Xcode beta test bundle path
• git diff --check