chore(vendor): vendorear ImPlot3D v0.4 como submodulo + target CMake

- submodule cpp/vendor/implot3d -> github.com/brenocq/implot3d, pinned
  commit 41ae3e447c0de20ecab95d38a4b4dc0835a3efc2 (v0.4).
- target CMake `implot3d` parejo a `implot` (3 sources: implot3d.cpp,
  implot3d_items.cpp, implot3d_meshes.cpp).
- fn_framework linkea implot3d para que cualquier app pueda usar las
  funciones viz/*_3d sin configurar nada extra.
- VENDORING.md externo (no tocamos el README upstream).

Issue 0028.

cpp/vendor/implot3d.VENDORING.md

@@ -0,0 +1,41 @@
+# ImPlot3D — vendoring notes
+
+3D plotting library for Dear ImGui by Breno Cunha Queiroz, sister project to
+[ImPlot](https://github.com/epezent/implot).
+
+- **Upstream:** https://github.com/brenocq/implot3d
+- **License:** MIT (see `cpp/vendor/implot3d/LICENSE`)
+- **Version:** v0.4
+- **Pinned commit:** `41ae3e447c0de20ecab95d38a4b4dc0835a3efc2`
+  (2026-04-05, _"chore: bump version to v0.4"_)
+
+The vendor lives as a git submodule in `cpp/vendor/implot3d` and is checked
+out at the pinned commit above.
+
+## Files compiled into the `implot3d` CMake target
+
+- `implot3d.cpp`
+- `implot3d_items.cpp`
+- `implot3d_meshes.cpp`
+
+`implot3d_demo.cpp` and the contents of `example/` are intentionally left
+out of the build — kept for reference only.
+
+## Update procedure
+
+```bash
+cd cpp/vendor/implot3d
+git fetch origin
+git checkout <new-commit>
+cd ../../..
+git add cpp/vendor/implot3d
+# Update the pinned commit in this file.
+git commit -m "chore(vendor): bump implot3d to <new-commit>"
+```
+
+## Used by
+
+- `cpp/functions/viz/surface_plot_3d.cpp` — `ImPlot3D::PlotSurface`
+- `cpp/functions/viz/scatter_3d.cpp` — `ImPlot3D::PlotScatter`
+- `cpp/framework/app_base.cpp` — creates/destroys the ImPlot3D context
+  alongside the ImPlot one.