send the native mtp option to the phone's model picker for bundles with an mtp head

[RaajeevChandran]

Summary

• /models/options now includes a Native MTP option (Off (AR) / On (Adaptive)) for local bundles with an MTP head, so the phone's model picker offers the same choice as the Mac composer's
• capability is read from the bundle on disk, off the main thread, the same check composer selection uses; bundles whose tuning blocks MTP only offer Off
• the option is the Mac's global Speculative Decoding setting, not a per-model choice, so a PUT writes through the same route as the composer and Settings, and null resets to Off
• the phone gets the choice without the composer's help text; the Mac picker is unchanged
• documented in docs/MOBILE_PROTOCOL.md §12.3

Changes

• Behavior change
• UI change (screenshots below)
• Refactor / chore
• Tests
• Docs

Checklist

• I have read CONTRIBUTING.md
• I added/updated tests where reasonable
• I updated docs/README as needed
• I verified build on macOS with Xcode 16.4+