fix: resolve retina backgrounds by terminal extension

[OskarEichler]

Summary

• derive @2x background names from the actual terminal extension
• preserve uppercase and mixed-case extensions
• avoid treating extensionless source files as their own retina companion
• add focused regressions

Why

The lowercase-only replacement did not change .PNG and extensionless paths. If the original file existed, it was then incorrectly passed to tiffutil as both the base and retina image.

Tests

• TEST_FILES=mac/dmgBackgroundTest corepack pnpm ci:test
• corepack pnpm compile
• corepack pnpm ci:validate
• git diff --check

Breaking changes

None.

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Latest commit: 3e5b981

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