Add Blender 4.2+ extension support (Blender 4.5 through 5.2)

[jmossymoss]

Package the add-on as a Blender extension so it installs cleanly on Blender 4.2 and newer, including 4.5 LTS and the 5.x series up to 5.2 LTS:

• Add blender_manifest.toml, the extension metadata used by the Get Extensions install flow. The zip keeps the add-on package in a single nested directory, a layout accepted by both the extension installer and the legacy add-on installer.
• Keep bl_info for legacy installs, replacing the removed url/wiki_url keys with doc_url and bumping the minimum Blender version.
• Drop pre-2.8 code paths (INFO_MT_file_import menu, 3D view background images, view_axis) that are dead on all supported versions.
• Set scene.camera on import. On Blender 2.8+ setting the 3D view's local camera alone does not make the imported camera the active one, so camera view and renders kept using the previously active camera.
• Set up the camera background image even when no 3D view area exists, and guard against a missing screen in background mode.
• Close the project file after parsing to stop ResourceWarnings.
• Fix build.py to run on Python 3 (len() of a map object raised a TypeError and raising a string is invalid), read the version from the manifest and include it in the built zip.
• Update the README install instructions for the extension workflow.

Claude-Session: https://claude.ai/code/session_01EGJUzQ4zuQdubTBWmT6nNv