ci(workflows): added RUSTFLAGS configuration for x86-64-v3 CPU optimization in CI and native builds

- Added RUSTFLAGS environment variable configuration to CI workflow for x86-64-v3 CPU optimization across multiple build jobs.
- Added per-matrix rustflags configuration for native builds on x64 platforms (Linux, macOS Intel, Windows).
- Updated build-native script to default to native CPU optimization for local builds when not cross-compiling.

packages/natives/scripts/build-native.ts

@@ -15,6 +15,11 @@ const isCrossCompile =
 	targetPlatform !== process.platform ||
 	targetArch !== process.arch;
 
+// Default to native CPU optimization for local builds; CI overrides via RUSTFLAGS env
+if (!isCrossCompile && !Bun.env.RUSTFLAGS) {
+	Bun.env.RUSTFLAGS = "-C target-cpu=native";
+}
+
 async function cleanupStaleTemps(dir: string): Promise<void> {
 	try {
 		const entries = await fs.readdir(dir);