feat(mobile): prototype Duo on the shared v5 navigator

[juliusmarminge]

Keep the Duo prototype separate from the general v5 migration in #12646. This draft moves the existing Duo work onto that shared navigator instead of maintaining a second split-stack/header implementation. The original v4 experiment remains in #12557.

This layer adds iOS 27.1 fold/occlusion geometry and vertical-bar traits to the native layout observer, reserves the active toolbar edge and fold around thread lists/composers, and binds UIBarButtonItem.axisBehavior through v5's neutral header. The RNS patch file is 1,340 lines here versus 1,290 in the shared migration. The difference includes diff context. Generic stack rendering, iPad columns, search/menu/toolbar/appearance bindings, modal presentation, and markdown resizing belong below this PR.

The same Expo app and React Navigation router remain in use. UIKit owns split-column collapse/expansion; the selected conversation and draft stay in the application model. The experimental split host is enabled on iPad and iOS 27.1; production device gating remains open.

The branch inherits the shared native header background/shadow bindings and custom-title geometry fix. Main already supplies a native label geometry guide so UIKit can recognize React custom titles when shaping automatic scroll-edge effects; the shared migration now ports that compatibility fix to v5's native header-content factory. It applies to custom title/subtitle content without T3 Code semantics or an extra blur API. The forced navigation-bar material workaround has been removed. The iOS 26 iPad sidebar also uses the automatic native effect, with top Settings/filter controls, floating search, and a darker inset background. Files loading and terminal Back retain one owner.

Validation: mobile typecheck and the four scroll-edge helper tests pass after this rebase; the existing focused navigation/layout/reserved-region suite passed 69 tests. A fresh Duo native build passes and the installed fingerprint matches. The current unfolded landscape client shows the selected review, preserved draft, automatic native header protection, and conversation/composer clearance from the toolbar rail with the 43-thread fixture. Navigation and screenshot scroll positioning use the existing callbacks/list API through CDP; these are not new physical touch or pose-transition checks. Earlier touch checks confirmed sidebar and filter callbacks but predate this revision. XCTest still targets the inactive display for some Duo interactions, so folded/book/portrait, keyboard, sheets, inspector transitions, and actual touch behavior need further checks on this revision.

This is a progress draft while the upstream native APIs settle. The shared iPhone header before/after, iOS 26 iPad automatic-effect capture, scroll recording, and native search/filter/Back checks are in #12646. All library patches should either fix generic library behavior or expose UIKit capabilities using the library's existing conventions.

Current unfolded landscape — native split columns, selected review, preserved draft, and system toolbar rail:

The sidebar scrolled under UIKit's automatic header protection:

Follow-up screenshot matrix:

Display and pose	Capture
Inner, flat landscape	Sidebar open and closed; conversation; Files/Git/terminal inspector
Inner, flat portrait	Sidebar open and closed; conversation; inspector
Inner, partially folded book, landscape	Thread list + conversation; composer near the fold; inspector
Inner, partially folded, portrait	Column arrangement; fold clearance; composer
Outer, closed portrait	Thread list with search/filter/compose; conversation with Back; settings and project-picker sheets
Outer, closed landscape, if supported	Thread list; conversation; sheets

For each supported pose, capture keyboard hidden and visible with a draft, then light and dark appearance. Include active search and an open filter menu; record one uninterrupted flat → book → closed → open transition to show draft and selection retention.

Built with GPT-6 in the Codex harness.

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

ℹ️ No successful main baseline artifact is available yet. This run establishes the initial measurement.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	—	13.5 KiB	—	15.1 KiB	✅
Codex	Thread snapshot wire	—	7.0 KiB	—	7.3 KiB	✅
Codex	Live turn WebSocket wire	—	6.5 KiB	—	7.8 KiB	✅
Codex	Live turn WebSocket decoded	—	56.2 KiB	—	66.4 KiB	✅
Codex	Live turn messages	—	9	—	21	✅
Claude	Total thread wire	—	13.5 KiB	—	15.1 KiB	✅
Claude	Thread snapshot wire	—	7.1 KiB	—	7.3 KiB	✅
Claude	Live turn WebSocket wire	—	6.4 KiB	—	7.8 KiB	✅
Claude	Live turn WebSocket decoded	—	57.0 KiB	—	66.4 KiB	✅
Claude	Live turn messages	—	9	—	21	✅

Baseline: unavailable · PR result: 5b715dc · Source CI: failure

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 114.0 KiB
• Claude decoded thread snapshot: 114.6 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.