refactor(effects): introduce WgslEffect for shader-only post-process effects

Replace boilerplate .h/.cc pairs for simple single-pass effects with a
generic WgslEffect base class that takes a shader string + optional
WgslEffectParams (binding 3). Port Flash, Passthrough, Heptagon, Scratch,
and GaussianBlur to thin header-only wrappers — no .cc files, no CMake
entries needed. Removes 5 .cc files (-243 lines).

Update EFFECT_WORKFLOW.md, CONTRIBUTING.md, and AI_RULES.md to document
the WgslEffect (Path A) vs full class (Path B) workflow. Doc cleanup:
fix stale GaussianBlurParams/PostProcessEffect references and test counts.

handoff(Claude): WgslEffect landed; 5 effects ported; docs updated.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/TODO.md b/TODO.md
index 25069d6..c89c9a6 100644
--- a/TODO.md
+++ b/TODO.md
@@ -23,7 +23,7 @@ Reduce weights from f16 (~3.2 KB) to i8 (~1.6 KB).
 
 ## Priority 3: Test Infrastructure Maintenance [ONGOING]
 
-**Status:** 34/34 tests passing
+**Status:** 35/35 tests passing
 
 **Outstanding TODOs:**