docs(player-info): document buffered dispatch validation

[ChechiDev]

Summary

• Updates docs/operations/player_info_buffered_warm_pool_smoke.md: replaces the placeholder "no live scraping" status section with full E2E validation results (Phases A0–J3, all PASS), including checkpoint summary, H-REPAIR process-group kill rationale, Phase I real-source soak results, and schema gotchas discovered during validation.
• Creates docs/architecture/buffered-dispatch-engine.md: architecture decision record establishing the buffered dispatch model as a shared always-active dispatch runtime with per-workload policies; documents reference primitives, validation evidence, and the critical G-phase concurrency caveat.
• Updates CHANGELOG.md: adds three entries under [Unreleased] for the completed validation and the architecture decision.

No source code, migrations, or tests were modified.

Test plan

• Verify only CHANGELOG.md, docs/architecture/buffered-dispatch-engine.md, and docs/operations/player_info_buffered_warm_pool_smoke.md are changed
• Confirm no secrets, DSNs, passwords, or credentials appear in the diff
• Confirm .gitignore is not staged