Add OpenGL renderer capability target

docs/modernization/roadmap.md

@@ -384,8 +384,12 @@ Status: started. `pp_renderer_api` exists as a headless renderer-neutral target
 with texture descriptor, byte-size, viewport, mesh, readback bounds, command
 context, render device, shader program descriptor, mesh, render target,
 readback, trace interface validation, and the canonical PanoPainter shader
-catalog now consumed by the legacy OpenGL app initialization path. OpenGL
-classes are not yet behind these interfaces.
+catalog now consumed by the legacy OpenGL app initialization path.
+`pp_renderer_gl` now exists as the first OpenGL backend library and owns pure
+OpenGL capability detection for framebuffer fetch, map-buffer alignment, and
+float texture support. The legacy app delegates extension interpretation to
+that backend library, but the existing renderer classes are not yet fully
+behind the renderer interfaces.
 
 Implementation tasks:
 
@@ -404,6 +408,8 @@ Implementation tasks:
   - `Sampler`
   - `ShaderManager`
   - `Shape`
+- Keep OpenGL runtime capability decisions in `pp_renderer_gl` with headless
+  tests before moving GL object lifetimes behind backend types.
 - Preserve current shader behavior and asset paths.
 - Add deterministic GPU tests:
   - clear
@@ -619,6 +625,10 @@ Results:
   aggregate stroke-script counts/distances.
 - `panopainter_validate_shaders` passed, validating 25 shader programs and 7
   shader includes for stage markers and include graph integrity.
+- `pp_renderer_gl_capabilities_tests` passed on default MSVC, vcpkg-headless,
+  and Android arm64 configure/build, covering framebuffer fetch, map-buffer
+  alignment, desktop GL core float support, GLES float/half-float extensions,
+  and WebGL exclusion behavior.
 - PowerShell analyze automation returns JSON summaries and includes the shader
   validation target.
 - `windows-msvc-vcpkg-headless` configured through the Visual Studio bundled