fix(mobile): prevent fallback tool timeout cascades

[cha1latte]

Summary

• keep the mobile original-file extractor alive at app root in a minimally visible 1x1 WebView
• coalesce concurrent same-book fallback extraction requests
• stop cascading fallback content calls for a turn after the source becomes unavailable
• add regression coverage for host ownership, provider behavior, request coalescing/retry, and the turn-level circuit breaker

Root cause

Android can freeze the extractor WebView when it is mounted at 0x0. Concurrent fallback calls then start duplicate extraction waits, and each one independently times out.

Verification

• TZ=UTC pnpm --filter @readany/core test — 588/588 passed
• pnpm --filter @readany/app-expo test — 20/20 passed
• pnpm --filter @readany/app-expo exec tsc --noEmit — passed
• Biome check on all 12 changed files — passed with existing noExplicitAny warnings only
• live Android proof in the downstream Shlai build: fallback search 1.415s, TOC 5ms, chapter context 4ms, with no timeout errors