chore(runtime): retire the test-only V1 HistoryCompactBlock path and Synthesis Cache #3215

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.