fix(ai): repair current chapter context

[cha1latte]

Summary

• resolve reader context from the loaded TOC so subsection anchors that share a document map to their logical parent chapter
• resync AI reading context when the TOC arrives after the latest relocate event
• reuse cached original-book chapters during repeated generic TOC reads instead of clearing the warm fallback first
• localize agent guardrail errors and remove hardcoded Chinese failure text

Verification

• pnpm --filter @readany/core test — 80 files, 590 tests passed
• pnpm --filter @readany/app-expo test — 6 files, 14 tests passed
• pnpm --filter @readany/app-expo exec tsc --noEmit — passed
• Biome check on all 16 changed files — zero errors
• Live Android proof: Desire at “Opening Up” resolved to Chapter 8 at 77.2%, answered in English, with no ragToc timeout