Untracked-file survey extracted and shared

Listing untracked files, filtering them, stat-ing each one and slicing off the over-budget tail is now one shared function returning candidates, a too-many-untracked result, or a failure, used by both directory sync inventory telemetry and the new sync offer probe. Files touched by a git clean filter, a working-tree encoding or an ident attribute are found via check-attr and excluded from cheap blob-id comparison; remaining hashes are batched through hash-object --stdin-paths.