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

backend/app/api/routes/flows.py

@@ -29,6 +29,7 @@ from app.flow.messages import qualify
 from app.flow.pipeline import ValidationIssue
 from app.flow.schemas import (
     NAME_PATTERN,
+    BrainGraph,
     FlowDef,
     FlowsPublic,
     FlowStatePublic,
@@ -264,6 +265,14 @@ def read_node_types() -> Any:
     return node_type_info()
 
 
+# Above "/{name}" for the same reason "library" is: a flow called "graph" would
+# otherwise be unreachable.
+@router.get("/graph", response_model=BrainGraph)
+def read_graph(controller: FlowControllerDep) -> Any:
+    """Every flow as one graph, with nodes talking to the same thing merged."""
+    return controller.brain_graph()
+
+
 # -----------------------------------------------------------------------------
 # Shared nodes
 #