Fix macOS Intel release support

[maxschmieder05-create]

Summary

Fixes the macOS build/runtime path so the app can run locally on Mac and so releases publish the correct Mac artifacts.

Root causes

• The release workflow only published VibeToText_<version>_aarch64.dmg, but the README said that ARM64 build would also run on Intel Macs under Rosetta 2. Rosetta works in the opposite direction: it runs Intel apps on Apple Silicon, not Apple Silicon apps on Intel Macs.
• Local macOS cargo check / tauri dev failed because Tauri validates bundle.resources paths before beforeBundleCommand creates the Windows DLL resource files. CI created placeholder stubs, but local Mac users did not.
• macOS used the static sherpa-onnx build. On this Mac, Moonshine warm-up crashed during ONNX Runtime session creation with an uncaught Ort::Exception. Switching macOS to sherpa-onnx shared matches the other platforms and loads Moonshine successfully.

Changes

• Add an x86_64-apple-darwin macOS release matrix entry on macos-13.
• Document separate Apple Silicon and Intel DMG downloads.
• Create bundle resource validation stubs from build.rs for local builds.
• Switch macOS sherpa-onnx from static to shared.
• Stage macOS .dylib runtime libraries and bundle them into Contents/MacOS.
• Add @executable_path rpath to the built macOS executable before bundling so dyld can find the staged dylibs.
• Update release/build docs to describe shared runtime library staging.

Validation

• Installed missing local prerequisites: Rust via rustup and CMake via Homebrew.
• Ran npm ci.
• Ran node --check across src/*.js and scripts/*.cjs.
• Ran cargo fmt --check.
• Ran cargo check --all-targets.
• Ran npm run tauri -- build --debug --bundles app.
• Launched the debug .app bundle directly with VIBE_TO_TEXT_SELF_TEST=1; Moonshine loaded and self-test passed 3/3 WAV fixtures.

Note: Tauri still emits an unrelated warning that the bundle identifier dev.vibetotext.app ends with .app.