backend-agents E20: shared event-apply consumer (applyTurnEvent)

[btipling]

Parent: #794
Umbrella: #794
Label: backend-agents
Status: DRAFT
Blocked on: #813 (E19)

Implements one row of the restart umbrella. One issue → one PR → one adversarial review → one merge → close. Do not expand scope.

Slice

Refactor runHarnessTurn's embedded onStreamEvent table into a single applyTurnEvent used by both legacy /api/agent and durable stream attach paths.

Behavior parity. Existing harnessChat / wasm-int locks stay green.

DoD

• One apply function, two producers
• typecheck + full vitest + wasm-int green
• No Wasm/protocol change

Refs

• Parent/umbrella: plan: backend-agents RESTART — client-independent turns (umbrella, ≥20 small issues + handoff) #794