MM-11: Expand ContextEngine with a Current Maka compatibility adapter

Parent

#771

Checkpoint

MC-3 ContextEngine ownership

What to build

Introduce the single ContextEngine assembly interface and wrap current replay, compact, search, retrieval, and trimming behavior without changing default semantics.

Acceptance criteria

• The adapter returns provider-safe messages plus diagnostics.
• Shadow comparison can run old and new assembly without double compact.
• Strategy id and config hash are present.
• Adapter exceptions produce typed fallback diagnostics.
• Existing callers can continue through the old path during expansion.

Required tests

• Trace replay equivalence
• fake strategy injection
• unknown strategy
• adapter failure
• multiple prepareStep calls

Benchmark

Current direct path vs compatibility adapter; target >=99% explainable trace equivalence.

Blocked by

• [MM-07] Validate provider shape and hard budget on every replay fallback #786 (MM-07)
• [MM-10] Export bounded recall traces through headless A/B artifacts #789 (MM-10)

Rollout and rollback

Keep the old path as default until surface migrations and equivalence gate pass.