Map renderer blend tokens to OpenGL

docs/modernization/build-inventory.md

@@ -210,7 +210,9 @@ Known local toolchain state:
   border-color parameter mapping used by legacy `Sampler`. The PanoPainter
   shader attribute binding catalog, shader stage tokens, compile/link status
   queries, active-uniform count query, and matrix-uniform transpose token used
-  by legacy `Shader` creation also live here. `Shader` no longer spells GL enum
+  by legacy `Shader` creation also live here. Renderer API blend factor/op to
+  OpenGL token mapping is tested here with explicit support flags so `GL_ZERO`
+  stays distinguishable from unsupported enum values. `Shader` no longer spells GL enum
   names directly. It also owns the PanoPainter shader uniform catalog and legacy hash
   mapping used by `Shader` active-uniform discovery and the uniform uniqueness
   check. App OpenGL initialization debug severity, debug output, GL info string,

docs/modernization/roadmap.md

@@ -467,7 +467,9 @@ interpretation to that backend library. Sampler wrap, min/mag filter, and deskto
 mapping for legacy `Sampler` also lives in `pp_renderer_gl`. The PanoPainter shader attribute
 binding catalog, shader stage tokens, compile/link status queries, active-uniform
 count query, and matrix-uniform transpose token also live in `pp_renderer_gl`
-and are consumed by legacy `Shader` creation. Shader uniform hashing, catalog
+and are consumed by legacy `Shader` creation. Renderer API blend factor/op to
+OpenGL token mapping also lives in `pp_renderer_gl`, with explicit support flags
+so `GL_ZERO` remains distinguishable from unsupported enum values. Shader uniform hashing, catalog
 validation, active-uniform mapping, and the legacy uniform uniqueness check now
 delegate to `pp_renderer_gl` as well. `Shader` no longer spells GL enum names
 directly. App OpenGL initialization debug severity, debug output, GL info