feat(mobile): prototype radial model picker

[WillysKitchen]

Below written by sol, but hope you like the concept, if there's anything you want me to change, I'm happy to point my tokens at it :)

Important

Experimental PoC — not ready to merge. This draft is open so maintainers can inspect, run, and discuss the interaction before we commit to production scope.

What Changed

• Adds an environment-gated radial model and reasoning picker to the expanded mobile composer.
• Supports hold, slide, and release selection, with the model fan anchored to the leading corner and reasoning centered over its trigger.
• Sources choices from the thread's live provider models and reasoning capability descriptor, then updates the real thread selection.
• Adds a pen action for an in-memory two-to-four-item palette and a conventional tap fallback.
• Locks the horizontal composer toolbar for the duration of a marking gesture.

Enable the experiment when starting Metro with EXPO_PUBLIC_MODEL_PICKER_PROTOTYPE=1.

Why

This explores whether a thumb-driven marking menu can make frequent model and reasoning changes faster without competing with iOS or Android edge gestures. It is intentionally an experimental PoC: palette membership is session-local and the existing picker remains unchanged when the flag is absent.

Not Production-Ready

• Palette membership is session-local and is not persisted.
• The marking interaction currently uses React Native Pressable; a production pass should evaluate a dedicated gesture recognizer and its cancellation semantics.
• The interaction still needs broader physical-device, accessibility, and ergonomics validation.
• Product direction and production architecture are intentionally left open pending maintainer feedback.

UI Changes

Before / after

Before	After

Interaction

t3-model-picker-pr-demo-compressed.mp4

Supporting states

Model fan	Dark mode	Palette editor

Verification

• ./node_modules/.bin/vp test run apps/mobile/src/features/threads/model-picker-prototype/modelPickerPrototypeState.test.ts
• ./apps/mobile/node_modules/.bin/tsc -p apps/mobile/tsconfig.json --noEmit
• ./node_modules/.bin/vp fmt --check <changed files>
• git diff --check
• Seeded iOS Simulator pass with the experiment enabled: model and reasoning fans, four-choice palette, tap fallback, palette editor, stationary-hold cancellation, and toolbar gesture lock.
• Seeded iOS Simulator pass without the flag: existing attachment, model, configuration, and send toolbar remains unchanged.
• The simulator automation available on iOS 26.3 cannot emit touch-move events, so the complete hold-slide-release path is covered by the prepared interaction recording rather than the final semantic automation pass.
• Claude Opus 5 review plus a final CodeRabbit CLI review of the revised diff; no unresolved automated findings.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

Built with GPT-5.6 Sol via Codex in T3 Code.

[coderabbitai]

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[cursor]

Cursor Bugbot has reviewed your changes using high effort and found 2 potential issues.

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[macroscopeapp]

Approvability

Verdict: Needs human review

1 blocking correctness issue found. This PR introduces a new feature (~1000 lines) with multiple unresolved review comments identifying bugs in palette state management and gesture handling. These substantive issues warrant human review before merging.

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[WillysKitchen]

Review update for 893f480ef:

• Fixed palette truncation by deduplicating IDs before applying the four-choice cap; added a focused regression test.
• Fixed stationary long-press commits by tracking actual finger travel separately from the menu anchor; a hold-and-release now cancels.
• Mounted the radial SVG overlay only while the menu is entering/open/exiting and hid it from the accessibility tree.
• Restored explicit model/reasoning accessibility labels and isolated the prototype provider icons from existing production callers.
• Kept the reconnect status pill stable on ordinary taps while preserving toolbar gesture locking.

Validation:

• Focused state tests: 2 passed.
• Mobile TypeScript check: passed.
• Formatting and git diff --check: passed.
• Seeded iOS Simulator: four-item palette and stationary-hold cancellation verified.
• Claude Opus 5 review completed; its two correctness findings are addressed. Its production recommendation to evaluate a dedicated gesture recognizer is documented in the draft limitations.
• CodeRabbit CLI re-review of the revised five-file diff: 0 findings.

All three inline review threads have been answered and resolved. This remains an experimental draft PoC, not a merge-ready feature.