fix(native): iOS swipe back like Slack's, tabs and channels 4-5x faster - #479

🟡 Confidence 6/10 · needs attention. Held at 6 because a correctness warning is open.
_{1 warning −1 · tests partial −1 · risk high −1}

Turns the swipe back into a whole-page drag over pictured layers with a projected commit, stops tab navigations from freezing the screen, and cuts Message cost by patching Foldkit's subscription fibers and caching the app's Shared. The open composer finding (F1) is not fixed: should_begin still tests direction only.

• SwipeBack::should_begin accepts a rightward drag from anywhere, not the left edge
• Commit uses the projected flick (PROJECTION, COMMIT_SHARE 0.5) instead of raw distance
• tab_pressed lets a tab's navigation draw with no cover over the page or bar
• Patched forkSubscriptionFibers derives all dependencies in one model fiber via queues

Still open from earlier reviews

• 🟠 Warning · F1 · Window-level swipe begins on rightward drags inside the composer · apps/native/host/ios/src/system.rs:617

_{8b2d989 · Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}