fix(session): make context usage persistence authoritative

[bobleer]

Summary

Follow-up to #2098 after review found that the UI-owned metadata write could race with title saves, miss non-Web hosts, survive history rewrites, and leave Peer gap recovery without an exact value.

• persist typed currentContextUsage in the Session runtime for model requests and applied compression
• serialize writes with the Session mutation owner and reject stale events from removed turns
• preserve or clear provenance across fork, rollback, delete, and partial-history hydration
• expose the authoritative value through restore_session_view for Peer recovery
• keep ACP context accounting separate and retain only safe single-round fallbacks

Validation

• Web UI focused suites: 157 passed
• Web UI type-check
• bitfun-core context usage tests: 5 passed
• services-core metadata contracts: 15 passed
• services-core lineage tests: 2 passed
• Desktop metadata merge contract
• repository hygiene and core boundary checks