fix(scene): cancel buffer creation for deactivated files

[dlyongemallo]

No description provided.

[Copilot]

Pull request overview

This PR fixes a performance/UX issue where buffer creation for diff files would continue even after the user navigated away, by checking the active flag at two points in create_buffer and short-circuiting early. The cancelled error is suppressed in the window's load_file to avoid confusing error messages.

Changes:

• Add early-exit checks in File.create_buffer before and after produce_data(), cleaning up allocated buffers and raising a sentinel CANCELLED error
• Change File.active default from false to true (previously never explicitly set to true), and add an early bail-out in Window.load_file for inactive files
• Add functional tests covering both cancellation scenarios

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
lua/diffview/vcs/file.lua	Add CANCELLED sentinel, default active to true, add two cancellation checks in create_buffer
lua/diffview/scene/window.lua	Skip load_file for inactive files, suppress CANCELLED errors
lua/diffview/tests/functional/file_spec.lua	Tests for both pre- and mid-produce_data cancellation
lua/diffview/tests/functional/window_spec.lua	Tests for window-level bail-out and mid-async cancellation

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