testsuite: cross-directory/temp/backup/dest coverage at depth

testsuite: cross-directory/temp/backup/dest coverage at depth

Fill the highest-restructure-risk gap: options that do two-directory / rename /
outside-tree work, asserted at >=3 levels deep with the aux tree kept outside
the main tree, and asserting the option's specific property rather than just
tree equality (which the ported tests already cover).

  alt-dest-deep  --link-dest hardlinks unchanged files (same inode), --copy-dest
                 copies (never links), --compare-dest omits unchanged files;
                 ref tree outside both src and dest.
  temp-dir       cross-dir temp->final rename at depth; temp dir left clean; a
                 missing --temp-dir fails (so the option is proven consulted).
  partial        --partial keeps the partial in the dest file; relative
                 --partial-dir stages per-directory at depth (pre-seed +
                 interrupt/resume); absolute --partial-dir writes the partial
                 outside the tree.
  inplace        --inplace keeps the destination inode across a delta update;
                 the default temp+rename path replaces it.
  append         --append completes truncated files tail-only; --append-verify
                 repairs a corrupted prefix (protocol >= 30).
  backup-deep    --suffix saves <name>S beside the new file; --backup-dir
                 relocates old files to a parallel deep tree outside the dest
                 and captures deletions under --delete.

All green on master and under --protocol=29/30.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>