[minor] Give NodeEditorRenderer a hook for host content in a node body

[matt-edmondson]

Fixes #441

What was wrong

RenderNode drew a fixed node body — a title bar, one ImGui.Text per input pin, one right-aligned ImGui.Text per output pin — with no callback anywhere on the type. A host wanting a value editable on the node face had two options and both were bad: fork the renderer, giving up zoom, hover highlighting, the measured position and dimension feedback and the pin offset publishing, all of which live in that class; or draw with raw ImNodes and keep NodeEditorEngine for topology and physics alone, which loses the same things.

ImNodes itself has no such restriction — anything submitted between BeginNode and EndNode is drawn in the node and sized into it.

The change

public Action<Node>? DrawNodeBody { get; set; }

Invoked inside each node, after its pins and before EndNode. Null draws nothing extra, so a renderer nobody configures draws exactly the node it always drew.

The placement is the load-bearing part. PublishPinOffsets measures each pin from a row RecordPinRow collects as that pin is submitted, and the engine uses those offsets to measure a link between the points it is actually drawn between. Running the callback after every pin row is recorded means nothing a host draws can move the point a link is drawn to. Moving the call above the pins makes DrawNodeBody_LeavesThePinPositionsWhereTheyWere fail, which is what that test is for.

The issue also sketches a per-pin DrawPinBody. That one is left alone deliberately: whether an input pin should offer an inline editor depends on whether it is connected, which is a question about the graph rather than about drawing, and the issue says it needs the most thought. This PR is the per-node half, which stands on its own.

Two things the doc comment and README state rather than enforce: the ID stack inside the callback is already the node's, since ImNodes pushes the node's ID as it begins it; and an exception thrown out of the callback escapes before EndNode and leaves the frame unusable, so a host that can fail should catch its own failures.

Tests

Three in NodeRenderingTests, which drives real frames through ImGuiAppHarness:

• DrawNodeBody_ReachesEveryNodeAndDrawsWithoutError — called once per node, a pinless node included, with ImGui.GetCurrentContext().ErrorCountCurrentFrame at 0
• DrawNodeBody_ContentIsSizedIntoTheNodeAndSettles — the node grows to hold the body and then holds still, rather than growing every frame
• DrawNodeBody_LeavesThePinPositionsWhereTheyWere — the published pin screen positions do not move when a body is drawn

Verified by reverting rather than assuming: removing the call site fails the first two (Expected:<3>. Actual:<0>, and the node not widening); moving the call above the pins fails the third ("A body drawn under the pins moved an output pin").

tests/ImGui.NodeEditor.Tests 101/101 pass on this branch, on .NET 10.0.401 / Linux.

Also adds a NodeEditorRenderer README row and a short usage section.

🤖 Generated with Claude Code

https://claude.ai/code/session_01P6bYWX7oFj1YTAAw9M516w

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Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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