Fix Windows path escape in generated Blender render script (#390)

Use repr() (via f-string !r) for path embedding instead of raw-string
r'...' prefixes. Raw strings are fragile with trailing backslashes and
quotes in paths; repr() produces a correctly-escaped Python literal
for any path.

Affects three sites in bpy_gen.py:
- HDRI image path in _gen_world_settings
- Output filepath in _gen_render_output
- 'Render complete:' print statement in _gen_render_output

Fixes #343