Worktree 0.15.0

• worktree archive (and finish --archive, and sweep) leaves cargo's own build layout out of
  an archive. Below a target recognised by discard-cache's structure (also when a tracked file
  below it hides it from discard-cache), an untracked file whose first component below the
  target is debug, release, .rustc_info.json, tmp, a Cargo-signed CACHEDIR.TAG or a
  profile directory (holding .fingerprint/; also <target>/<triple>/<profile>/) is neither in
  dirty.patch nor in the fingerprint. Tracked files below the target stay archived. Everything else below the target, such as
  target/ess-conformance/, stays archived. Such an archive is the new manifest format
  worktree.archive/3, which records per target the files and bytes left out as build_output;
  the command prints left out <files> file(s), <bytes> bytes of cargo build output under <target>. An archive that left nothing out is written byte for byte as 0.14.0 writes format 1
  or 2, and both stay readable.
• Removal through a format 3 archive (gc --apply) deletes each left-out file as cache only after
  re-observing it, immediately before deletion, as build layout below the same recognised target.
  A left-out path that is no longer build layout, or layout below a target the archive did not
  record, refuses as archive-stale.
• worktree prune-archives --strip-build-output [--repo] [--scope repo|profile] [--id]... [--dry-run|--apply] removes build layout from archives written before. A dry-run, the default,
  streams each dirty.patch and lists the bytes and sections it would strip, what stays, and the
  prune verdict afterwards. Only whole sections that purely add an untracked file (new file mode, a C-unquoted path no other section names and HEAD's tree does not track) whose path is
  build layout below a directory the patch itself recognises as a cargo target (a Cargo-signed
  CACHEDIR.TAG or a profile's .fingerprint/; tmp/ only beside a profile) are stripped; both
  halves of a type change, other tracked changes, undecodable paths and nested images stay. --apply --id verifies the new patch over
  HEAD in a scratch index, recomputes worktree_tree, replaces the patch (or deletes it) and writes
  the format 3 manifest last, then prints the bytes freed. It refuses an invalid manifest,
  unrecorded content, a present tree, a gone repository or an unusable patch and leaves that
  archive unchanged; it never deletes an archive. On 2026-10-10, 19.18 GiB of one machine's
  31.52 GiB of UncommittedState archives was build output under target/.