replaced pre-built binary with automated Rust build script

Removed the committed tc_helper.framework/tc_helper binary and
replaced it with an automated Xcode build phase ('Build and Embed
Rust Framework') that compiles the Rust library from source on
every build using cargo.

- Added Frameworks/tc_helper.framework/tc_helper to macos/.gitignore
- Build phase runs cargo build --release --target aarch64-apple-darwin,
  packages the output as a .framework, and embeds it into the app bundle
- Errors with a clear message if Rust/cargo is not installed

Author: srishtiii28

macos/.gitignore

@@ -5,3 +5,6 @@
 # Xcode-related
 **/dgph
 **/xcuserdata/
+
+# Rust-compiled native framework (built automatically by the Xcode build phase)
+Frameworks/tc_helper.framework/tc_helper

macos/Frameworks/tc_helper.framework/tc_helper

@@ -364,25 +364,25 @@
 			shellPath = /bin/sh;
 			shellScript = "echo \"$PRODUCT_NAME.app\" > \"$PROJECT_DIR\"/Flutter/ephemeral/.app_filename && \"$FLUTTER_ROOT\"/packages/flutter_tools/bin/macos_assemble.sh embed\n";
 		};
-		B47EAE0DA7514E2B9140D50B /* Embed Rust Framework */ = {
+		B47EAE0DA7514E2B9140D50B /* Build and Embed Rust Framework */ = {
 			isa = PBXShellScriptBuildPhase;
+			alwaysOutOfDate = 1;
 			buildActionMask = 2147483647;
 			files = (
 			);
 			inputFileListPaths = (
 			);
 			inputPaths = (
-				"$(SRCROOT)/Frameworks/tc_helper.framework/tc_helper",
 			);
-			name = "Embed Rust Framework";
+			name = "Build and Embed Rust Framework";
 			outputFileListPaths = (
 			);
 			outputPaths = (
 				"$(BUILT_PRODUCTS_DIR)/$(FRAMEWORKS_FOLDER_PATH)/tc_helper.framework/tc_helper",
 			);
 			runOnlyForDeploymentPostprocessing = 0;
 			shellPath = /bin/sh;
-			shellScript = "set -e\n\n# Build the Rust library if the framework binary doesn't exist or source is newer\nRUST_DIR=\"$SRCROOT/../rust\"\nFRAMEWORK_SRC=\"$SRCROOT/Frameworks/tc_helper.framework\"\nFRAMEWORK_BIN=\"$FRAMEWORK_SRC/tc_helper\"\nRUST_LIB=\"$RUST_DIR/target/aarch64-apple-darwin/release/libtc_helper.dylib\"\n\nif [ ! -f \"$FRAMEWORK_BIN\" ] || [ \"$RUST_DIR/src/api.rs\" -nt \"$FRAMEWORK_BIN\" ]; then\n  echo \"Building Rust tc_helper library...\"\n  export PATH=\"$HOME/.cargo/bin:$PATH\"\n  rustup target add aarch64-apple-darwin 2>/dev/null || true\n  cargo build --release --target aarch64-apple-darwin --manifest-path \"$RUST_DIR/Cargo.toml\"\n  mkdir -p \"$FRAMEWORK_SRC\"\n  cp -f \"$RUST_LIB\" \"$FRAMEWORK_BIN\"\n  install_name_tool -id \"@rpath/tc_helper.framework/tc_helper\" \"$FRAMEWORK_BIN\"\nfi\n\n# Embed the framework into the app bundle\nFRAMEWORK_DST=\"$BUILT_PRODUCTS_DIR/$FRAMEWORKS_FOLDER_PATH/tc_helper.framework\"\nmkdir -p \"$FRAMEWORK_DST\"\ncp -f \"$FRAMEWORK_BIN\" \"$FRAMEWORK_DST/tc_helper\"\ncodesign --force --sign \"$EXPANDED_CODE_SIGN_IDENTITY\" --preserve-metadata=identifier,entitlements \"$FRAMEWORK_DST/tc_helper\" 2>/dev/null || true\n";
+			shellScript = "set -e\n\nRUST_DIR=\"$SRCROOT/../rust\"\nFRAMEWORK_DIR=\"$SRCROOT/Frameworks/tc_helper.framework\"\nRUST_LIB=\"$RUST_DIR/target/aarch64-apple-darwin/release/libtc_helper.dylib\"\n\nexport PATH=\"$HOME/.cargo/bin:$PATH\"\n\nif ! command -v cargo &>/dev/null; then\n  echo 'error: cargo not found. Install Rust from https://rustup.rs' >&2\n  exit 1\nfi\n\nrustup target add aarch64-apple-darwin 2>/dev/null || true\ncargo build --release --target aarch64-apple-darwin --manifest-path \"$RUST_DIR/Cargo.toml\"\n\nmkdir -p \"$FRAMEWORK_DIR\"\ncp -f \"$RUST_LIB\" \"$FRAMEWORK_DIR/tc_helper\"\ninstall_name_tool -id \"@rpath/tc_helper.framework/tc_helper\" \"$FRAMEWORK_DIR/tc_helper\"\n\nFRAMEWORK_DST=\"$BUILT_PRODUCTS_DIR/$FRAMEWORKS_FOLDER_PATH/tc_helper.framework\"\nmkdir -p \"$FRAMEWORK_DST\"\ncp -f \"$FRAMEWORK_DIR/tc_helper\" \"$FRAMEWORK_DST/tc_helper\"\ncodesign --force --sign \"$EXPANDED_CODE_SIGN_IDENTITY\" --preserve-metadata=identifier,entitlements \"$FRAMEWORK_DST/tc_helper\" 2>/dev/null || true\n";
 		};
 		33CC111E2044C6BF0003C045 /* ShellScript */ = {
 			isa = PBXShellScriptBuildPhase;