[Feature]: sign: cap package discovery without losing newest-first ordering

Is your feature request related to a problem? Please describe.

winapp.sign currently discovers MSIX/APPX packages with an uncapped recursive glob and stats every match so it can sort packages newest-first. On very large workspaces this can do more filesystem work than the QuickPick needs. This behavior predates the executable/library discovery work for #77 and is not caused by that feature.

Describe the solution you'd like

Investigate a bounded or incremental package-discovery strategy that preserves the current newest-first UX and cancellation behavior while avoiding a full workspace scan/stat pass when only a limited number of QuickPick results will be shown.

Additional context

Surfaced as out-of-scope feedback during review of the #77 executable/library QuickPick enhancement. Replacing the glob directly with capped vscode.workspace.findFiles would lose deterministic newest-first ordering, so the follow-up should preserve that behavior.