Draw every flow as one graph, merged on what it talks to

A node type can now say which outside thing its parameters point at, and
nodes sharing one — a broker topic, a URL, a bucket — are drawn as a single
neuron on a new /brain canvas. That makes the wiring which runs between
flows through a broker visible for the first time; no single flow's canvas
can show it. The key is read off stored parameters, so a credential
reference never reaches an id.

Layout is a d3 force simulation settled once and then frozen, lit by the
socket the editor already listens to: a neuron pulses when any node behind
it publishes, and its connections light as values pass.

Fixes the message pulse while here: interpolating the stroke against the
edge's `color-mix()` resting colour went through oklab and left the gamut,
which turned every pulse on both canvases fluorescent yellow.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017MeiWk3Yq12n2pTvnQWYvt

frontend/package.json

@@ -40,6 +40,7 @@
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
     "cmdk": "^1.1.1",
+    "d3-force": "^3.0.0",
     "form-data": "4.0.5",
     "lucide-react": "^0.562.0",
     "monaco-editor": "^0.56.0",
@@ -64,6 +65,7 @@
     "@playwright/test": "1.58.0",
     "@tanstack/router-devtools": "^1.142.11",
     "@tanstack/router-plugin": "^1.140.0",
+    "@types/d3-force": "^3.0.10",
     "@types/node": "^25.0.10",
     "@types/react": "^19.2.7",
     "@types/react-dom": "^19.2.3",