Remove SprigUI, add a BaseOS tab

[Sundownersport]

Summary

• Removes SprigUI — the repo is archived now that spruceOS supports the Miyoo Mini Flip properly. Clean deletion; it was referenced nowhere outside config.rs.
• Adds BaseOS (pvaibhav/BaseOS) as an install target — a third-party minimal OS for Anbernic RG XX devices, credited and linked as such in the tab's info text.
• Bumps to 1.6.0 (Cargo.toml, Info.plist).

Tabs are now Stable / Nightlies / TwigUI / BaseOS.

Why this isn't just a config entry

BaseOS ships .img.zip — a raw disk image inside a zip container. The installer classified anything ending in .zip as an archive to extract onto a formatted card, so a BaseOS install would have formatted the SD card and copied a 1.4 GB .img file onto it instead of burning it: an unbootable card, with no error shown.

So this also teaches the burn path about .img.zip:

• is_raw_image_asset() in logic.rs now matches .img.zip, replacing three drifting inline copies of the check in logic.rs and ui.rs.
• burn.rs gains open_image_reader(), which streams .gz, .zip or a bare .img through one path — replacing four copy-pasted gzip blocks. A worker thread owns the ZipArchive and hands 1 MB chunks over a bounded channel, since by_index borrows the archive and can't escape into the Box<dyn Read> the burn paths expect.
• Zip records its uncompressed size in the headers, so unlike .gz it needs no full-file pre-scan.
• .img.zip is ordered before .zip in strip_extensions and the auto-select priority list, since every .img.zip name also ends with .zip.

Read is deliberately fully qualified in the new code. One of the existing per-function use std::io::Read; imports is cfg-gated and guards the statement that follows it — consolidating them silently breaks the Windows build.

Verification

All four workflows green on all platforms: beta-add-baseos-tab.

Selection logic dry-run against each tab's live release, using the mappings parsed out of the committed config.rs:

Stable     v4.3.4          assets= 1  auto-select  -> ARCHIVE (format+extract)
Nightlies  v4.3.5-20260816 assets= 1  auto-select  -> ARCHIVE (format+extract)
TwigUI     0.5.0           assets= 1  auto-select  -> BURN (raw image)
BaseOS     v1.1.0          assets=11  SELECTION UI -> BURN (raw image)

All 11 BaseOS devices map to friendly labels, none falling through to a raw filename.

⚠️ Not runtime tested

CI proves this compiles; nothing has actually burned a card. The zip reader, the header size lookup and the read-back verification have never executed against a real image. Because the shared burn path changed, TwigUI (.img.gz) should be smoke-tested alongside BaseOS before this is trusted in a release.

🤖 Generated with Claude Code