Add local HTTPS staging for Arch Linux ARM mirrors - #3

Add a standalone Python 3.9+ command to stage Arch Linux ARM repositories locally over HTTPS. It defaults to aarch64 core, extra, alarm, and aur, using repository databases as package manifests without third-party Python dependencies.

The command verifies package sizes and SHA-256 on downloads and incremental runs, preserves package signatures, and publishes metadata only after all selected packages verify and upstream databases/signatures remain unchanged. It includes stage locking, disk-space checks, bounded concurrent downloads, optional pruning, JSON status reports, and per-repository freshness markers.

Malformed databases and unrecognized .files-only entries fail by default. Explicit recovery options can retain verified packages from an all-NUL record's exact .files counterpart or permit a named orphan, but report degraded status (exit 3), withhold metadata/freshness publication, and skip pruning. Preserving signatures does not itself authenticate them; that remains a pacman/keyring validation step.

Validation:

• Full local aarch64 inventory: core 311, extra 12,895, alarm 75, aur 12 — 13,293 packages, 50.59 GiB of package data and approximately 51 GiB on disk. The staging filesystem started with 963 GiB free.
• Observed a rolling upstream update during the first pull: metadata publication was withheld and completed packages were reused on rerun. The latest full recovery run verified all 13,293 packages with zero downloads and correctly exited degraded.
• A normal core/alarm/aur run verified all 398 packages with zero downloads and updated only those repositories' freshness markers.
• All 22 integration tests passed locally on Python 3.9 and 3.14, including an isolated pacman consumer test. Python 3.9, Python 3.14, and Arch/pacman CI jobs passed at 9f0bf38.

Remaining upstream blockers: Florida's extra.db still has an all-NUL findnewest-0.3-4/desc; its readable .files counterpart allows package verification but does not repair pacman metadata. extra.files also references qemu-system-cris-9.1.2-1-aarch64.pkg.tar.xz, absent from .db and returning HTTP 404. A normal full sync remains blocked pending upstream correction; the staged tree is not certified ready to serve.

This adds ARM staging, tests, CI, documentation, and ignore rules only. Existing x86_64 and T2 mirror workflows, production services, and R2 uploads are unchanged. Downloaded packages and local reports are Git-ignored.