feat(shaders_lab): visual node editor (imgui-node-editor) + multi-source

- cpp/vendor/imgui-node-editor: vendorized thedmd/imgui-node-editor v0.9.4
- cpp/functions/gfx/dag_node_editor: new visual pipeline editor replacing dag_panel
- DagStep: source_ids[4] + editor_pos + editor_uid (multi-input support)
- DagNodeDef: num_inputs explicit; circle reclassified as Op
- dag_compile: N inputs per node, topological ordering preserved
- main: use node editor; destroy on shutdown
- patch imgui_extra_math.inl: guard operator*(float, ImVec2) for imgui >= 18955

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cpp/functions/gfx/dag_node_editor.md

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+---
+name: dag_node_editor
+kind: function
+lang: cpp
+domain: gfx
+version: "1.0.0"
+purity: impure
+signature: "bool dag_node_editor(std::vector<DagStep>& pipeline)"
+description: "Renderiza el node editor visual (imgui-node-editor) para el DAG de shaders. Modifica el pipeline in-place: añade/borra nodos, gestiona aristas (source_ids). Devuelve true si la topologia cambio."
+tags: [dag, imgui, node-editor, shader, visual, pipeline, gfx]
+uses_functions: []
+uses_types: []
+returns: []
+returns_optional: false
+error_type: "error_go_core"
+imports: ["imgui_node_editor.h", "gfx/dag_catalog.h", "gfx/dag_types.h"]
+tested: false
+tests: []
+test_file_path: ""
+file_path: "cpp/functions/gfx/dag_node_editor.cpp"
+params:
+  - name: pipeline
+    desc: "vector de DagStep que representa el pipeline actual; modificado in-place"
+output: "true si la topologia cambio (nodos o aristas añadidos/quitados); false en caso contrario"
+---
+
+## Notas
+
+Sustituye a `dag_panel` como UI del pipeline. Usa `ax::NodeEditor` (thedmd/imgui-node-editor v0.9.4) con contexto singleton estático.
+
+Codificacion de IDs:
+- Node id = `editor_uid` del step
+- Output pin = `(uid << 8) | 0`
+- Input pin = `(uid << 8) | (slot + 1)`
+- Link id = `(from_uid << 20) | (to_uid << 8) | slot`
+
+Aplica topological sort (Kahn) tras cada cambio de topologia. Rechaza ciclos en `BeginCreate`.
+
+Multi-source: cada nodo declara `num_inputs` (0-4). Los slots de `source_ids[]` se mapean a los inputs del shader GLSL (`a`, `b`, `c`, `d`).
+
+## Dependencia
+
+Requiere `imgui_node_editor` static library linkeada (`cpp/vendor/imgui-node-editor`).