Add Core integration SDK boundary

[LARIkoz]

What changed

• add the versioned eidetic.engine JSONL worker and canonical v1 schemas;
• define the one-way Core / integration SDK ownership boundary;
• expose logical-index-only sync, reconcile, search, rerank, capabilities, and health operations;
• document the future Core-owned durable ingestion contract and its fail-closed readiness gates.

Why

The integration SDK is now a separate repository. Core must publish the
canonical worker and schemas without exposing storage paths or importing SDK
code. Durable writes remain Core-owned and no executable ingestion surface is
introduced.

Scope safety

This branch was rebuilt from origin/main by cherry-picking only the three
reviewed SDK-boundary/ADR commits. It contains none of the unrelated local M3
commit series or dirty M3 worktree files.

Checks

• Core worker contract: 9/9
• Engine contract: 7/7
• dedicated MLX suite: 16/16
• git diff --check
• changed-path scan: zero M3 paths