[minutechreview]

In the loopback port forwarder, error() set socket.data.closed = true without lowering the listener's connection count. When close() ran next, it saw closed and returned early, so the count stayed at 1 and sweep() never closed that listener. Enough errored connections and a machine runs out of its 64 preview port slots.

• backend/src/runners/loopback.ts: one releaseConnection helper marks the socket closed, lowers the count and bumps lastActivity, and only does this once per socket. close() and error() both call it. close() still ends the link stream, error() still resets it.
• backend/src/runners/loopback.test.ts: new case that drives a socket through open, error, then close (the Bun.listen handlers are captured with spyOn), moves the clock past FORWARDER_IDLE_MS, and expects sweep() to close the listener.

Testing:

• bun test src/runners/loopback.test.ts: 4 pass. With loopback.ts from main, the new case fails (sweep() returns 0, expected 1).
• bun test src/runners src/runs/terminal.test.ts: 58 pass.
• backend tsc --noEmit (TypeScript 5.9.3) is clean.

Fixes #304

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