Track possible future Moonshine Base STT support

[BumpyClock]

Summary

Track Moonshine Base as a possible future local STT model option.

Current decision

No current plan to ship Moonshine Base support. It should not be documented or tested as part of the current supported-model contract.

If revisited later

• Add a ModelId variant and string/serde coverage.
• Add catalog metadata and ordering.
• Decide whether it is built in, downloadable, or plugin-backed.
• Wire engine/download/runtime support.
• Add UI and model contract tests.

Context

Raised while triaging PR #19 review feedback. Current code has no Moonshine implementation; existing support references should be removed from current docs/instructions.