Route pylink-square-mcp through J-Link Remote Server backend

[emrainey]

The pylink-square MCP server crashes periodically. Root cause hypothesis: every tool call creates a fresh pylink.JLink(), opens USB, connects, and discards -> repeated DLL load/unload + USB churn in a long-lived process.

Goal:

1. Make J-Link Remote Server on localhost (default port 19020) the default connection backend for all scripts in tools/pylink-square-mcp.
2. mcp_server.py should launch JLinkRemoteServer (-Port, optional -USB <serial/nickname>) on startup if it is not already listening, then persist across MCP restarts.

Per SEGGER KB (https://kb.segger.com/J-Link_Remote_Server), JLinkRemoteServerCLExe takes -Port and -USB (S/N or nickname) options; device/speed stay client-side in pylink connect(). Client connects via jlink.open(ip_addr='127.0.0.1:').

[emrainey]

Implemented on branch issue-52 → PR #53.

• All 13 J-Link tools converted to connect via a persistent localhost J-Link Remote Server (port 19020) with direct-USB fallback.
• mcp_server.py now launches JLinkRemoteServer -Port <port> [-USB <serial>] on startup if not listening and leaves it running across MCP restarts.
• Added --device/--speed/--remote-port/--usb-serial CLI opts with JLINK_MCP_* env fallbacks.
• Verified live on hardware through the tunnel (debug_target, run_for, rtt_read --continuous); host LLVM build + 18/18 unit tests pass. No C++ touched.