[Feature] macOS audio support and build targets

[lqdev]

Summary

Add macOS (x86_64-apple-darwin, aarch64-apple-darwin) as a supported build target. This includes CI workflows, build scripts, and testing CoreAudio output via rodio.

Related: #131 (Meta-Epic: Cross-Platform Audio Playback)

Problem / Background

The project currently builds for:

• Windows: x86_64-pc-windows-msvc, aarch64-pc-windows-msvc
• Linux: x86_64-unknown-linux-gnu, aarch64-unknown-linux-gnu

macOS is not supported despite rodio supporting CoreAudio via cpal. The CI workflow (.github/workflows/release.yml) has no macOS job. No scripts/build-macos.sh exists.

Proposed Solution

1. CI Workflow

Add a build-macos job to .github/workflows/release.yml:

build-macos:
  runs-on: macos-latest
  strategy:
    matrix:
      target: [x86_64-apple-darwin, aarch64-apple-darwin]
  steps:
    - uses: actions/checkout@v4
    - uses: dtolnay/rust-toolchain@stable
      with:
        targets: ${{ matrix.target }}
    - run: cargo build --release --target ${{ matrix.target }}
    - run: cargo test --target ${{ matrix.target }}

2. Build Script

Create scripts/build-macos.sh following the pattern from scripts/build-linux.sh:

• Auto-detect architecture (x86_64 vs arm64/aarch64)
• Build with release profile
• Create .tar.gz archive with checksums
• Support universal binary via lipo (optional, stretch goal)

3. Packaging (Stretch)

• Homebrew formula for brew install podcast-tui
• .dmg disk image with drag-to-Applications
• Code signing with Apple Developer ID (requires cert)

4. Documentation

• Update docs/BUILD_SYSTEM.md with macOS instructions
• Update README.md install section
• Add macOS prerequisites (Xcode Command Line Tools)

Files to Modify

File	Change
.github/workflows/release.yml	Add build-macos job
scripts/build-macos.sh	New — macOS build script
docs/BUILD_SYSTEM.md	Add macOS section
README.md	Update install instructions

Acceptance Criteria

• CI builds macOS x86_64 and aarch64 binaries
• Release artifacts include macOS tarballs with checksums
• rodio audio output works on macOS (CoreAudio)
• Build script auto-detects architecture
• Documentation updated

Implementation Notes

• macOS GitHub Actions runners are more expensive (3x Linux cost) — consider only running on release tags, not PRs
• aarch64-apple-darwin cross-compilation on x86_64 runner requires Xcode and cargo build --target
• rodio/cpal CoreAudio backend should work without extra dependencies on macOS
• Test on both Intel and Apple Silicon if possible

[m13v]

the build-macos job runs cargo build + cargo test, which verifies the binary links against CoreAudio. but the acceptance criterion 'rodio audio output works on macOS' is a runtime gate, not a compile one: GitHub's macos runners are headless with no output device, so cpal can enumerate zero devices and opening a default output stream fails at runtime. a green CI proves it builds against CoreAudio, it can't prove playback works. that line needs a real device or a mocked sink to actually check, otherwise it passes silently and breaks on a user's machine.