Content-addressed events validate invariants on deserialize

[flyingrobots]

Salvaged from old Continuum crates/jitos-core/src/events.rs, preserved only in git history (formerly listed in GoodIdeas.md, now deleted).

Idea: content-addressed events should validate their invariants on deserialize, not just on construction.

• policy belongs in structure, not metadata
• canonical payload wrapper types beat trusting raw bytes
• prefer explicit event classes over loose blobs
• IDs should be boring and content-addressed

Scope: this is an Echo / git-warp runtime-internal discipline, not a Continuum-owned contract. Filed here to preserve the idea before deleting the salvage note; move it to the owning runtime repo when it is picked up.

[flyingrobots]

Re-homing to Echo: this is runtime-internal event validation discipline, not Continuum coordination/profile truth.