Wire v2: responsive connection reader and bounded single writer #172

Problem

A long request on a subscription connection must not block unrelated pushes, and concurrent requests must not race session/KG changes.

Evidence

Source: 5c1f313379a5cb2995a43a66ddba0b5cc9798b7f (inspected 2026-10-03). These are source findings; the acceptance scenarios below still need implementation and execution.

• src/protocol/rest/handlers/ws.rs:645: Subscription completions are polled by the connection event loop.
• src/protocol/rest/handlers/ws.rs:925: Subscription pushes use the shared connection sender.

Acceptance criteria

• Use an owned reader, bounded in-flight execution set and single ordered writer; document barriers for KG/session mutations.
• Block a query for 500 ms while an external writer changes subscribed rows; deltas still meet the measured delivery budget.
• Test KG switch plus insert ordering, overload and disconnect cleanup with bounded tasks/queues.
• Pass saturated mixed query/write/delta measurements in Performance: baseline and mandatory pre-PR regression pipeline #163.

Delivery

Plan item: W02. R2 - Atomic programs + wire protocol.

Dependencies / related work: #171.

Contributes to #132; completing this slice alone does not close the broader issue.