fix(build): restore /O2 /DNDEBUG for native deps clobbered by cmake-rs on MSVC

cmake-rs hardcodes opt_level(0) and replaces CMake's Release C flags, so
ggml/whisper.cpp compiled at /Od — root cause of the W1 CPU-pathology
(RTF ~20) in docs/benchmarks/2026-07-parakeet-vs-whisper.md. cc-rs appends
env CFLAGS/CXXFLAGS last, restoring optimization for all native deps.

.cargo/config.toml

@@ -0,0 +1,10 @@
+# Fix: whisper.cpp (whisper-rs-sys) is configured by the cmake crate, which
+# hardcodes opt_level(0) and overwrites CMake's default MSVC Release flags
+# (/O2 /Ob2 /DNDEBUG) with unoptimized ones — ggml's CPU kernels were compiled
+# at /Od with asserts on, making CPU-only transcription ~40x slower than real
+# time (bench finding W1, docs/benchmarks/2026-07-parakeet-vs-whisper.md).
+# cc-rs appends these env flags after its own, so they win for every native
+# dep. MSVC-only flags are fine: WhispAssist is a Windows-native app.
+[env]
+CFLAGS = "/O2 /DNDEBUG"
+CXXFLAGS = "/O2 /DNDEBUG"