Add support for NUNIF_HOME environment variable

[Copilot]

Adds NUNIF_HOME environment variable support for controlling where pretrained models, configs, cache, temp files, and lock files are stored. Useful for packaging or read-only source directories.

Changes

• nunif/utils/home_dir.py: New module providing home_dir() that returns NUNIF_HOME (with ~ expansion) or falls back to source directory
• iw3: Updated model downloads, GUI config, cache, and lock file paths
• cliqa: Updated pretrained model paths
• stlizer: Updated SuperPoint model path (also fixed incorrect path)
• waifu2x: Updated model and cache paths
• README.md: Added documentation for NUNIF_HOME

Usage

export NUNIF_HOME=~/.config/nunif
python -m iw3.download_models
python -m iw3.gui

Or inline:

NUNIF_HOME=~/.config/nunif python -m iw3.gui

If unset, behavior is unchanged from current version.

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[nagadomi]

Operation mistake