Add FlagGems single-op perf benchmark harness

pytest plugin set + driver script for compiler A/B perf comparison:
- reproducible runs: fixed seed, yaml-driven shapes, autotune record/replay
- per-shape ttgir dump of actually-used variants with readable naming
- cudagraph-based timing with documented fallback semantics

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,2 @@
+runs/
+__pycache__/