test(scripts): lock package-inventory behavior with characterization tests

package-inventory compares archsetup's declared packages against the live system but had no tests, so a future archsetup edit (a new for-loop shape, a renamed install helper) could silently break the extraction.

I added two env seams so the script is testable without the real system. PKGINV_ARCHSETUP points the extractor at a fixture installer, PKGINV_PACMAN swaps in a fake pacman serving controlled query output. Both default to the real targets, so normal use is unchanged, and the seams match the env-override pattern audit-packages.sh already uses.

The 7 tests pin the extraction (direct calls, for-loop lists, variable-arg skip) and both diff directions against the fixture, with no network or real pacman db. I also added a make package-diff target so the tool is reachable alongside the test targets.

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