Save reusable autorouting phase JSON artifacts in CLI builds

[seveibar]

Automatically save completed CLI autorouting phases as importable pcbTracePaths JSON under .tscircuit/autorouting-artifacts/<entrypoint>/, and print each saved file's location. Users can copy a generated file into their project and pass its default JSON import to <autoroutingphase pcbTracePaths={savedPaths} /> without manually extracting routes.

The exporter resolves PCB port selectors, converts board coordinates to the phase group's local frame, preserves wire/via dimensions, and orders multi-terminal routes for replay. Artifacts are separated by entrypoint, subcircuit, phase, and stage, including parallel builds. Unsupported geometry or write failures produce a warning without failing the build; partial saved-path arrays are not written.

This applies to tsci build and commands using the shared CLI circuit renderer. Browser rendering in tsci dev is not included. Usage and limitations are documented in docs/autorouting-artifacts.md.

Validation:

• 7 feature tests pass, including an actual routed board and parallel CLI builds with same-named entrypoints.
• TypeScript checking, CLI build, and formatting pass.
• Complete-route and fanout artifact replay verified against current core during development.
• Existing autorouter diagnostics tests could not load on the rebased checkout because the local sharp native binary is missing.