feat(gfx): mesh_obj_load — minimal Wavefront .obj parser

mesh_obj_parse (pure) + mesh_obj_load (impure file helper).
Soporta v / vn / f (tris y quads). Genera normales per-face si
faltan (flat shading). Quads se parten en 2 tris; n-gons (>4) se
descartan silenciosamente. Indices 1-based positivos y negativos.

issue 0029

cpp/functions/gfx/mesh_obj_load.h

@@ -0,0 +1,29 @@
+#pragma once
+
+#include <cstddef>
+#include <cstdint>
+#include <vector>
+
+namespace fn::gfx {
+
+// CPU-side mesh: positions interleaved with normals stride 3 floats each,
+// indexed by `indices`.
+struct Mesh {
+    std::vector<float>    positions;  // x,y,z stride=3 (count = num_vertices*3)
+    std::vector<float>    normals;    // x,y,z stride=3 (same length as positions)
+    std::vector<uint32_t> indices;    // tri-list (count multiple of 3)
+};
+
+// Parse Wavefront .obj from a text buffer. Pure: no I/O.
+// Supports: v, vn, f (triangles and quads). Ignores: vt, mtllib, usemtl, o, g, s.
+// If the file has no vn, normals are generated per-face (flat shading).
+// Quads in `f` are split into 2 triangles (fan). N-gons (>4 verts) are not supported.
+//
+// Returns an empty Mesh ({}, {}, {}) on parse failure (no vertices found).
+Mesh mesh_obj_parse(const char* obj_text, size_t len);
+
+// Read file from disk and parse. IMPURE (file I/O).
+// Returns empty Mesh on read or parse failure.
+Mesh mesh_obj_load(const char* path);
+
+} // namespace fn::gfx