MUXUE-72: optimize AI context reads

[musnows]

Summary

• implement SVE-4-006 by paging story_index directory fields and loading insights only for the returned chapter IDs
• implement SVE-4-007 by reading conversation counts, compaction boundaries, and uncompacted tails separately
• reuse one conversation-tail snapshot across context estimation, message building, generation, and compaction
• keep full conversation history APIs unchanged and add regression coverage for ordering, boundaries, appends, and large fixtures

Verification

• npm run typecheck
• npm run test:unit (500 tests)
• integration suite split into bounded Vitest groups (374 tests)
• npm run test:system (77 tests)
• npm run build
• isolated SQLite PRAGMA integrity_check and PRAGMA foreign_key_check
• isolated production health check at /api/health

The aggregate npm test process was interrupted by the local resource limit without a Vitest assertion failure; the same 951 unit, integration, and system tests passed in bounded invocations.

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2 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
scriverse	Ignored	Preview	Aug 14, 2026 9:58am
scriverse-showcase-demo	Ignored	Preview	Aug 14, 2026 9:58am