refactor: namespace public convert_units() as dedx_convert_units() #112

Part of #108 (health-review epic).

Problem

convert_units() (declared in include/dedx_tools.h, defined in src/dedx_tools.c) is the only exported public symbol that lacks the dedx_ prefix. An un-prefixed global symbol named convert_units is a likely collision for any application or other library linked alongside libdedx, and it breaks the otherwise-consistent naming of the public API.

Proposed change

• Rename to dedx_convert_units().
• Keep a thin deprecated convert_units() alias for one release (documented as deprecated) so existing consumers don't break immediately.
• Update examples, tests, docs, and the Python binding accordingly.

Acceptance criteria

• dedx_convert_units() is the documented public name.
• A deprecated convert_units() alias remains for backward compatibility (with a doc note + removal target).
• All in-repo callers (examples/tests/bindings/docs) use the new name.
• No other public symbol in include/ lacks the dedx_ prefix.