Restore alpha snapshot update channel

[serrebidev]

Summary

• restore the Alpha snapshots channel as an explicit update mode above Disable updates
• use an alpha-shaped fallback version when checking alpha from stable/beta builds, because the NV Access API returns no alpha update for stable version strings
• migrate the old Disable updates index to the new index so existing disabled configurations stay disabled

Testing

• python C:\tmp\test_update_channel_modes.py
• python C:\tmp\test_nvda_update_api_modes.py
  • stable: update metadata returned
  • beta: update metadata returned
  • alpha direct from stable version: no update returned
  • alpha fallback: alpha-56053,b9ab7106 -> https://download.nvaccess.org/snapshots/alpha/nvda_snapshot_alpha-56053,b9ab7106.exe
  • disabled: local mode; no update API request expected
• py -V:Astral/CPython3.13.13 -m py_compile addon\globalPlugins\updateChannel.py buildVars.py
• python -m ruff check addon\globalPlugins\updateChannel.py
• scons
• scons pot
• git diff --check