Engagement deep module — generic like/message/notify on the registry

[jrnxf]

This was generated by AI during triage.

From architecture review candidate #2 (recommendation: Worth exploring). Captured for triage; not yet specced to agent-ready.

Problem

Roughly 20% of the messages, reactions, and notifications modules is the same like / message / message-like CRUD re-typed per entity, expanded ~7× rather than compressed into a module. The pattern is documented in CLAUDE.md as a convention but has never been made into a deep module.

Proposed solution

On top of the EntityRegistry (#6, now landed), add generic helpers parameterized by entity type — e.g. likeRecord(type, id), listMessages(type, …), notifyEngagement(type, event) — each resolving specifics through the registry rather than a per-type branch.

Benefits

• ~1500 loc across the three modules → ~600. Depth rises: small interface, lots of behavior behind it.
• Tests target the generic helper once instead of 7 near-identical paths.

Relationships

• Depends on EntityRegistry — one module owns the engagement entity map #6 (EntityRegistry) — the prerequisite is now in main, so this is the natural next chain.
• Triage should check overlap with the recently-merged slices EntityRegistry slice 2 — rewire likes through the registry (all 13 types) #8/EntityRegistry slice 3 — rewire messages through the registry (all content types) #9/EntityRegistry slice 4 — rewire notification owner-resolution through the registry #10, which already moved per-type dispatch onto the registry — this candidate is the generalization step on top of that.