perf(trajectory): bound live stream persistence overhead

[jubaoliang]

Summary

• aggregate assistant/tool stream snapshots in memory and persist at boundaries instead of every token
• throttle live snapshots, bound subscriber queues, and cache/throttle metrics aggregation
• record context references instead of duplicating prompt and workspace content
• preserve final usage across tools/cancellation and harden SSE resume
• document trajectory APIs and cap list summaries

Verification

• make all — 2891 passed, 8 skipped
• pre-commit (make precommit) — 2891 passed, 41 skipped
• targeted trajectory unit/integration suite — passed
• Ruff and strict mypy — passed