Fix hook priority conflicts and standardize on_Binary naming

on_Snapshot priority fixes:
- redirects.bg.js stays at 31, staticfile.bg.js → 32
- headers.js stays at 55, readability.py → 56
- mercury.py → 57, htmltotext.py → 58

on_Binary hooks now have numeric priorities:
- 10: npm_install.py
- 11: pip_install.py
- 12: brew_install.py
- 13: apt_install.py
- 14: custom_install.py
- 15: env_install.py