fix(ci): install image test dependencies and make contracts portable

[lukisch]

The baseline CI stopped collecting image tests because Pillow was missing. Install it in the existing regression step. Once collection succeeded, CI exposed Python3.10 tomllib import, an asset filename with wrong Git case, and two Windows shortcut tests depending on a real Windows SystemRoot/notepad.exe. Add conditional test-only tomli, use the actual tracked SoftwareCenter.ico spelling, and exercise Windows percent-variable expansion against real temporary files using ntpath on every host. The file checks remain real; the first case now also rejects the deleted target. Runtime code, version, skips, matrix and existing policy remain unchanged. Baseline CI36245702422; first repair CI38812791709 identified these latent failures. Local final full suite on Windows Python3.12/offscreen Qt: 270 passed,5 existing skips; full Ruff and diff checks pass. Exact-current-head CI remains required. No Store, deployment or runtime mirror acceptance. Author model: gpt-6.1-sol (Codex). Independent model review pending. Draft only; merges disabled.

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