tools: add physics-sanity + Docker MCP scaffolding + tools/README

- tools/physics-sanity/ — JS library of cross-node balance helpers
  (mass / hydraulic / hydraulic-power / oxygen-transfer / energy) with
  7 unit tests + a CLI demo. Designed for `require()` from per-node
  integration tests where shape-based unit tests miss physically-
  impossible plant states.
- tools/docker-compose.yml + tools/mcp/{node-red-admin,influxdb,browser}
  scaffolding — placeholder Dockerfiles + a ROADMAP.md for the Node-RED
  admin MCP. Compose file is the target shape for the Q3-2026 migration
  to the central MCP server; the per-service Dockerfile stays in this
  repo as the canonical definition either way. Implementations are TODO.
- tools/README.md — top-level tooling index; documents the CI order for
  running every tool on a PR.
- .gitignore: ignore tools/.env (developer-specific MCP endpoints).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tools/mcp/browser/Dockerfile

@@ -0,0 +1,5 @@
+# placeholder — see ../README.md status table
+FROM node:20-alpine
+WORKDIR /app
+RUN echo 'console.error("mcp-browser not yet implemented"); process.exit(1);' > server.mjs
+CMD ["node", "server.mjs"]