libSceAppContent technical debt

sceAppContentDownloadDataGetAvailableSpaceKb (libSceAppContent) - a title whose param.json declares downloadDataSize 0 gets 0 KB and OK; the console's behaviour for a title without a download data area is unverified (it may return an error instead). KytyPS5 does not special-case it, and shadPS4 stubs 1048576 KB
sceAppContentGetAddcontDownloadProgress (libSceAppContent) - imported by PPSA08755, which fails to load without it; added without an implementation because its behavior is unverified. SharpProspero lists (uint32_t serviceLabel, entitlementLabel, SceAppContentAddcontDownloadProgress*), but no PS5 reference gives the progress layout or the error codes
sceAppContentDownloadDataFormat (libSceAppContent) - empties the /download0 directory, creating it if needed. Imported by PPSA01317. A null mount point returns PARAMETER as sceAppContentTemporaryDataFormat does, and another mount point throws as in sceAppContentDownloadDataGetAvailableSpaceKb; the console's error codes for both and its behaviour with files still open are unverified

Additional content (libSceAppContent) is not installed or mounted: sceAppContentAddcontMount answers NOT_FOUND for every entitlement label and sceAppContentAddcontUnmount for every mount point, as no add-content mount is ever handed out; sceAppContentGetAddcontInfo answers DRM_NO_ENTITLEMENT for every label and sceAppContentGetAddcontInfoList reports 0 entries; sceAppContentAddcontEnqueueDownload answers PARAMETER for a null label and NOT_FOUND otherwise, since no download is ever queued. The signature follows SharpProspero; no reference gives the console's error codes, so both are unverified
Temporary app-content storage tracks one /temp0 mount, preserves its files on unmount, and rejects new path resolutions until remount. Mount, format, space-query and unmount errors follow fpPS4; the pointer ABI is also declared by SharpProspero. Existing open host descriptors remain usable after unmount; console behaviour with open files is unverified. Host filesystem failures still throw.