Integrate COG_WHEEL env var into StandardGenerator
Refactor installCog() to dispatch based on GetWheelConfig():
- installEmbeddedCogWheel() for COG_WHEEL=cog (default when cog_runtime: false)
- installEmbeddedCogletWheel() for COG_WHEEL=coglet
- installCogletAlpha() for COG_WHEEL=coglet-alpha (default when cog_runtime: true)
- installWheelFromURL() for COG_WHEEL=https://...
- installWheelFromFile() for COG_WHEEL=/path/to/file.whl
Backwards compatible: existing behavior unchanged without COG_WHEEL set.
Closes cog-1xb
