Enhancement suggestion

Current behavior: Custom lines can only be added to a scene via mjGEOM_LINE geoms in the scn->geoms[] buffer, which shares the fixed maxgeom limit with all other visualization geometry. Applications that need many lines (LOD grid overlays, force vectors, debug annotations) quickly exhaust the geom budget.

Desired behavior: A separate, dynamically-growing line buffer in mjvScene with a simple push API:

mjv_resetLines(&scn);  // clear per frame
mjv_addLine(&scn, from, to, rgba);  // add as many as needed
// ... mjr_render draws them with depth testing

Lines would be rendered by MuJoCo's OpenGL pipeline with proper depth testing, blending, and occlusion against scene geometry. No maxgeom limit.

Why this is useful:

• LOD wireframe overlays on height fields (water simulation, terrain)
• Force/velocity vector visualization at scale
• Debug annotations for robotics (contact normals, joint axes) without stealing geom slots
• Any application where line count is data-dependent and unbounded

Proof of concept: Working implementation at https://github.com/stanbot8/mujoco/tree/feature/unbounded-line-buffer

Changes (5 files, ~100 lines):

1. mjvisualize.h: Add nline, maxline, linevert, linergba to mjvScene
2. mujoco.h: Declare mjv_addLine and mjv_resetLines
3. engine_vis_init.c: Implement allocation, free, reset, and dynamic-grow add
4. engine_vis_init.h: Internal declarations
5. render_gl3.c: Render pass after transparent geoms, before labels, with depth test and blending

The buffer starts at 1024 lines, doubles on overflow, and is freed by mjv_freeScene. mjv_resetLines just sets nline = 0 (no reallocation).

Tested with a multi-scale ocean simulation demo that draws LOD grid wireframes (coarse + fine + sub-cell) on a height field surface. Lines correctly occlude behind a floating sphere and the water surface itself.

Happy to add tests and follow the style guide if there is interest. This is independent of (but complementary to) issue #3181 (per-geom wireframe).

Platform: Windows 10, MuJoCo latest main branch