Revert default backend to CPP; use parameterized benchmarks instead

DIFF_ENGINE doesn't yet support all atoms, so keep CPP as default.
The matrix_stuffing benchmarks now use ASV parameterized benchmarks
to compare both backends side-by-side.

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

Author: Transurgeon

cvxpy/settings.py

@@ -196,7 +196,7 @@
 DIFF_ENGINE_CANON_BACKEND = "DIFF_ENGINE"  # C diff engine backend for parameter application
 
 # Default canonicalization backend, pyodide uses SciPy
-DEFAULT_CANON_BACKEND = DIFF_ENGINE_CANON_BACKEND if sys.platform != "emscripten" else SCIPY_CANON_BACKEND
+DEFAULT_CANON_BACKEND = CPP_CANON_BACKEND if sys.platform != "emscripten" else SCIPY_CANON_BACKEND
 
 # DPP parameter threshold for auto-selecting COO backend
 # When problem is DPP and total parameter size >= this threshold, use COO backend