Fix macOS build: add backend/models directory and constrain dependencies

[Copilot]

macOS release workflow failed due to missing backend/models directory referenced in PyInstaller spec and unconstrained Python dependencies causing yanked package installation.

Changes

Directory structure

• Add backend/models/.gitkeep - PyInstaller spec line 28 requires this directory at build time (models download to ~/Library/Application Support/HeartMuLa/ at runtime)

Dependencies (requirements_macos.txt)

• Constrain transformers>=4.35.0,<4.57.0 - avoids yanked 4.57.0
• Add triton>=2.0.0; platform_machine != "arm64" - torch.compile optimization, skip on Apple Silicon
• Document optional dependencies (mamba-ssm, tensorboard)

Validation

• Add validate_build_assets.sh - pre-flight checks for files, directories, dependency configuration
• Update build/macos/README.md - validation usage and troubleshooting

Build artifacts

• Update .gitignore - exclude test scripts and build artifacts

Context

PyInstaller spec includes these data paths that must exist at build:

datas += [('backend/models', 'backend/models')]
datas += [('backend/generated_audio', 'backend/generated_audio')]
datas += [('backend/ref_audio', 'backend/ref_audio')]

Only backend/models was missing from the repository.

Original prompt

Observed Issues:

During the "Build macOS Release" workflow, multiple errors were encountered:

1. Missing Files:

   • build/macos/HeartMuLa.icns was not found after the generate_icon.sh script execution.
   • build/macos/codesign.sh was missing.
   • .github/DMG_README.txt was missing.
   • backend/models directory or one of its expected files was not found.

2. Python Dependency Issues:

   • Modules triton, mamba_ssm, and tensorboard were not found during the PyInstaller process.
   • A yanked version of the transformers library (4.57.0) was used, leading to potential setup issues.

Fixes Implemented in This PR:

1. Addressing Missing Files:

   • build/macos/generate_icon.sh: Verified and ensured the script exists and is executable. If missing, added it to the repository.
   • build/macos/codesign.sh: Added a placeholder script for code signing and ensured it's executable. For production, this should be updated with a valid signing script.
   • .github/DMG_README.txt: Added a preliminary DMG README file with instructions for the disk image.
   • backend/models: Verified the required files. If generated dynamically, updated the workflow to ensure proper generation and placement.

2. Python Dependency Updates:

   • Updated requirements_macos.txt to add missing dependencies:

     triton
     mamba-ssm
     tensorboard
     

   • Reverted to a stable version of the transformers library:

     transformers>=4.35.0,<4.57.0
     

3. General Workflow Improvements:

   • Added steps to validate the existence of dynamically generated files before execution.
   • Improved logging on any missing files to capture the output of related scripts and workflows.

This pull request was created from Copilot chat.

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