Refine Maven bootstrap script

[merrily01]

Is your feature request related to a problem? Please describe.

Describe the solution you'd like

1. Support for MVN_HOME:
   Users can now set MVN_HOME to point to an existing Maven installation. The script will validate it and skip downloading if it satisfies the required version.

2. Fix Maven installation detection:

Previous logic checked if the $3 variable was non-empty.

Now the script checks whether the binary path exists and is executable, providing a more reliable validation.

3. Download logic improvements:

Unified curl/wget handling with clear logs and fallback mechanism.

Ensures downloaded tarballs are reused if already present.

4. Code readability and logging

Describe alternatives you've considered

Additional context