- Remove LegacyBackport from classifypkg and webicached; canonical values
now flow through storage untouched
- Add legacyFieldBackport() in storage/legacy.go, called only at export time
(go: armv6→arm, ffmpeg windows: .gz/.empty→.exe)
- ExportLegacy now takes pkg name and returns LegacyDropStats (variants + formats dropped)
- fsstore.Commit logs dropped assets so filtering is visible
- Add FormatAPK (.apk) and FormatAppImage (.AppImage) to buildmeta and classify
so those files are properly classified and then correctly dropped from legacy export
rather than passing through as empty-format