[xcv58]

Summary

Fix custom Derived Data paths that could be split at spaces or incorrectly prefixed with the checkout directory.

• Honor environment and command-line overrides for DERIVED_DATA and TEST_DERIVED_DATA, preserving separate app/test defaults.
• Handle quoted relative and absolute paths consistently across app/CLI builds, tests, installation, Launch Services, and CI compatibility checks.
• Use existing shell/installer path normalization, document override precedence, and isolate native tools in the Make tests.

Validation

• Build-path tests: 3 passed with both current Python and Apple's bundled Python.
• Existing plugin-cache tests: 9 passed, including isolation, product selection, locking, and cancellation.
• git diff --check passed.

Local build result

• Passed full Debug app and CLI build (make build build-cli) at commit 27687bb0 on November 6, 2026.
• Xcode 27.1 (27A9275), macOS arm64, with code signing disabled.
• Used an absolute Derived Data path containing spaces; completed in 11m 38s.
• Verified the generated app and CLI executables. The native SDK contains both arm64 and x86_64 libraries.
• App installation, launch, and runtime smoke testing were not performed.