Feat: Write mesh overlay as a separate <stem>_mesh image

--draw-mesh no longer modifies the primary render.  The clean HDR FITS and
tone-mapped image are written first, then the final image-plane mesh is
overlaid in place on the already-consumed HDR buffer and saved as a
<stem>_mesh sibling.  This applies uniformly to single-frame, movie-frame,
and imported lens-map renders, with derived paths validated before catalog,
spacetime, and render initialization.

Dummy PSF backends still write nothing and report ignoring --draw-mesh.  The
reference-image target now uses the configured image extension so the
ENABLE_PNG=0 test path passes, and the CLI tests cover clean-main identity,
sibling naming, PATH_MAX overflow, and zero-tolerance HDR comparison.

README, README.zh-CN, and usage.md describe the new naming; the mesh reference
asset is renamed to match.

usage.md

@@ -355,9 +355,12 @@ count and each time slab's activation and terminal-ray summary.
 Movie renders always print one summary per time slab; `--verbose` also prints
 the ray counts before each slab is loaded.
 
-Pass `--draw-mesh` to alpha-composite image-plane triangle edges as
-one-pixel-wide 0.5 linear-gray diagnostic lines at 0.5 opacity. The line
-rasterizer uses coverage-based antialiasing.
+Pass `--draw-mesh` to also write the final image-plane triangle mesh as a
+`<output-stem>_mesh.png` sibling (`.ppm` in non-PNG builds). The main
+tone-mapped image and any `--hdr-output` FITS file remain mesh-free. The
+overlay alpha-composites image-plane triangle edges as one-pixel-wide 0.5
+linear-gray diagnostic lines at 0.5 opacity. The line rasterizer uses
+coverage-based antialiasing.
 
 ## HDR output