[contracts] Defer CursorRegistry until indexer identity spec stabilizes

[FlowmemoryAI]

Objective

Track CursorRegistry as a deferred contract concept until the indexer identity and observation identity specs are stable.

Context

A registry for cursors or indexer progress may be useful later, but premature contract design could bake in the wrong identity model. Issues around canonical FlowPulse observation identity and verifier/indexer status vocabulary should stabilize first.

Scope

• Define prerequisites for considering CursorRegistry.
• Identify which indexer identity facts must be off-chain, observed, verified, or committed.
• Do not implement CursorRegistry in this issue.

Acceptance Criteria

• Prerequisites and blockers are documented.
• CursorRegistry remains out of contract scope until indexer identity semantics are accepted.