test: support focused desktop E2E runs

[Azganoth]

Summary

• Add strict focused selection for already-built desktop E2E targets.
• Preserve fixture setup, isolated state, fresh processes, failure artifacts, cleanup, and port checks for selected runs.
• Expose persistence as an ordered write-and-restart target and document the explicit rebuild loop.

Related Issue

Closes #494

Verification

• pnpm test:e2e:desktop:runner verifies full-suite selection, independent target selection, the persistence dependency, and invalid input handling.
• pnpm check:frontend verifies formatting, TypeScript, linting, production build, and frontend coverage.
• pnpm build:e2e:desktop builds the packaged E2E binary.
• The pre-commit and pre-push hooks passed formatting, TypeScript, and linting checks.

Not verified: the focused Diagnostics target and ordinary full runner both reached the packaged Diagnostics scenario but failed its existing aria/Diagnostics visibility assertion; each retained normal failure evidence and cleanup, so the full packaged suite could not complete.