chore(runtime): retire the test-only V1 HistoryCompactBlock path and Synthesis Cache

Found by a repo-wide simplification audit; re-verified with `git grep -aI`. Independently confirmed by two separate audit passes.

The V1 `HistoryCompactBlock` artifact compaction path and the Synthesis Cache are test-only:

- The sole production composer of `AiSdkBackend` (`packages/runtime-host/src/server/execution-model-composition.ts`, checkpoint wiring around `:385`) supplies only `loadHistoryCompactCheckpoint` / `summarizeHistoryCompact` / `recordHistoryCompactCheckpoint`; it never supplies `loadHistoryCompact` / `writeHistoryCompact`.
- All 9 repo-wide producers of `writeHistoryCompact:` are in `packages/runtime/src/__tests__/ai-sdk-backend.test.ts`.
- `history-compact-artifacts.ts` and `synthesis-cache-artifacts.ts` have no production callers (tests + package export only).
- `MAKA_CONTEXT_SYNTHESIS_CACHE=on` is exposed (`context-budget-policy.ts`) but there is no production persistence capability behind it — a toggle that cannot do anything.
- `docs/architecture/llm-compaction-events-log-projection-draft.md` still describes V1 artifacts as a read-compat path, contradicting the actual wiring.
- The only real capability user is the benchmark script `scripts/deepseek-live-cost-baseline.mjs` (itself orphaned; see the audit-residue issue).

Suggested cleanup: delete the two artifact adapters + tests + exports; move or migrate the benchmark's legacy usage; drop the unreachable branches and the false toggle; fix the doc. If shipped artifacts/local DBs may still hold V1 blocks, a minimal decoder can be retained — but not the full write path.