fix(desktop): give isolated dev service a fixed port - #52013

Desktop dev (OPENCODE_DESKTOP_ISOLATED_SERVER=1) now starts its service on 0x0C0C (3084, "OC OC") instead of --port 0.

Why

Service.ensure() relies on the port as the single-instance lock: a second contender hits EADDRINUSE, recognizes the incumbent, and exits. Port 0 never collides, so two services can boot against the same opencode-local.db, and one dies with SQLiteError: database is locked:

sequenceDiagram
  participant D as Desktop ensure()
  participant A as serve --service (A)
  participant B as serve --service (B)
  D->>A: spawn
  Note over A: cold start > 5 s
  D->>B: spawn lock probe
  A->>A: bind :random ✅
  B->>B: bind :random ✅ (no collision)
  A--xA: boot: database is locked
  D--xD: "Background service failed to start"

Seen 13 times in one local log since 2026-08-21. Every occurrence had two serve --service --port 0 processes running at once: 5 started ~5 s apart (lock probe), 8 started within 3 ms of each other.

With a fixed port, the loser takes the existing path in packages/cli/src/server-process.ts: Service.incumbent / recognizeIncumbent → exit.

• 3084 is outside the Windows ephemeral range (49152–65535), so the OS doesn't hand it out to outgoing connections.