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

NOTEPAD.md

@@ -75,6 +75,8 @@ is what M4 still waits on, together with porting the flows.
 - PERF/UI: `useParamSuggestions` fetches every flow's detail to build the suggestion list. An aggregate endpoint if an installation ever has many flows.
 - CHORE/UX: the derived-cron chip also appears on the delay node, where `interval` is a rate limit rather than a schedule. May want it inject-only.
 - CHORE/UX: free-form params (python nodes) get no suggestions, since there is no schema to key them off.
+- FEAT/UI: the brain graph draws connections as plain lines, so which way a value travels is not shown. An arrowhead needs the path shortened to the target circle's rim, which needs the node radius on the edge.
+- CHORE/UI: fixing the yellow message pulse gave `edge-pulse` an explicit `to`, so a *selected* edge now decays to `--muted-foreground` and snaps back to its blue at the end instead of decaying into it. Barely visible, and only while an edge is selected.
 
 ### Infrastructure