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

ROADMAP.md

@@ -179,6 +179,12 @@ React + Vite, primarily desktop but usable on mobile. See `docs/architecture/str
       over what it has been doing all day — throughput and failure charts, a
       per-flow table, the recent cascades, failures that expand to their
       traceback, dead-lettered work and the audit trail
+- [x] Brain graph: every published flow on one canvas, with nodes that talk to the
+      same outside thing — a broker topic, a URL, a bucket — drawn as a single
+      neuron, so the wiring that runs between flows through a broker is visible at
+      all. Laid out by a force simulation settled once and then frozen, lit by the
+      same socket the editor listens to, and read-only: a neuron leads back to the
+      flow it came from
 - [x] Mobile-friendly canvas: touch connect, full-screen node panel
 - [ ] Installable as a PWA (`vite-plugin-pwa`)