🧭 [SPEC] [DAPR] Dapr blackboard atomicity and concurrency hardening

Why

PR #414 can ship an initial Dapr-backed blackboard store after the public API blockers are fixed, but the deeper distributed-write semantics need a separate hardening pass. Artifact/index atomicity, TTL exceptions, encrypted backend fallbacks, and ETag conflict retries should be explicit before users rely on Dapr as a shared blackboard backend.

Scope

• Define a Dapr blackboard guarantee matrix for transactional, TTL, encrypted, and non-transactional modes.
• Decide when artifact data and index updates can be committed in one execute_state_transaction.
• Document and test the weaker TTL path where artifact metadata forces a separate save_state.
• Implement ETag conflict retries by re-reading current state before retrying.
• Add failure-injection tests for partial writes, transaction failures, ETag conflict success/exhaustion, and reconciliation behavior.
• Update docs and examples so durability claims match implementation.

OpenSpec

• Change: openspec/changes/harden-dapr-blackboard-atomicity/
• Related initial PR: #414

Out of Scope

• DaprStateSessionStore
• Dapr Pub/Sub, workflows, actors, bindings, or service invocation
• Broad replacement of the BlackboardStore contract

Acceptance Criteria

• Dapr docs include a clear guarantee matrix.
• Transactional no-TTL publish can commit artifact and indexes atomically, or the design documents why not.
• TTL/encrypted/non-transactional paths have tested and documented reconciliation semantics.
• ETag retry behavior is wired into real read-modify-write paths and tested for success and exhaustion.