MCP stdio: write NDJSON so Cursor can discover search_docs

[alex-16moro]

Summary

Cursor MCP stdio expects newline-delimited JSON. The docs server was writing LSP Content-Length frames, which broke live tool discovery (search_docs).

• _write_message now emits one JSON object per line
• Reader still accepts Content-Length and NDJSON
• --selftest parses NDJSON
• overlay/mcp.optional.json uses workspace-relative .cursor/mcp-diffusers-docs.py (not /workspace/...)

Default overlay mcp.json stays empty ({}).

Convention gate

Tooling only. examples/scaffolded_scheduler unchanged.

Tests run

printf '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05"}}\n{"jsonrpc":"2.0","id":2,"method":"tools/list"}\n' | python3 -u .cursor/mcp-diffusers-docs.py
python3 tools/docs_mcp_server.py --selftest
python3 -m unittest discover -s tests -t .
python3 -c "import yaml"

Handshake prints two bare JSON lines, search_docs is listed, no Content-Length. 43 tests, 4 skipped. PyYAML imports on /usr/bin/python3.