feat: add ordered model library directories

[PhilipJohnBasile]

Adds ordered model-library directories so one MTPLX installation can discover models across a configured primary cache and additional user-selected roots. Discovery preserves root order and deduplicates references; resolution, downloads, updates, diagnostics, retrieval, Forge, CLI onboarding and the native picker use the same configuration.

The current-main integration preserves external-volume free-space measurement, safe download destinations, stale-copy recovery and newer configuration decoding. A malformed primary value or an invalid item in the additional-root list no longer loses valid roots or unrelated settings.

Current validation

Head 9740fbb2a0b079ad09fd1cdec0cfbd60759422cb merges upstream 21be78b3f51820eecef020e5e4855c0715eaf9a5. GitHub reports mergeable/clean and all four hosted checks pass.

• 1,196 focused Python tests passed across configuration, discovery, downloads/updates, CLI, retrieval and serving.
• The 956-test CI Python selection passed locally and the hosted no-MLX check passed.
• Native suites: 902 core tests with two skips, plus 39 host tests; zero failures on Apple Swift 6.4 / macOS 27.
• Changed-file Ruff and the PR diff whitespace check passed.

The native test framework was staged inside the isolated build directory. The user's existing checkout and model files were not modified by validation. Maintainer review and the merge decision remain outstanding.