pkg provider #816

Epic: #815 — Providers. The pkg provider (pkg/op/provider/pkg) and the platform drivers it fronts
(pkg/platform): apt, brew, port, winget, and the composite router. Every pkg task, fix, and chore files here.

Known members at filing: #813 (the constructor refuses the canonical purl form), and the winget driver defects found
on the 2026-09-05 Windows deploy.