install.sh: merge PERSONAL_DIRS contents instead of wholesale replace

The previous deploy_one() did `mv $HOME/<dir> $BACKUP_DIR/<dir>` then
`cp -a $YADR_DIR/<dir> $HOME/<dir>` for every entry in PERSONAL_DIRS.
For dirs the repo only partially populates (notably .local — repo only
tracks .local/bin/), this swept away unrelated user data: the most
recent re-bootstrap moved ~/.local/share/fonts/ (Intel One Mono, Open
Gorton, Roboto Mono, GALLAUDET, code128) into the timestamped backup,
making them appear missing.

Rework deploy logic:
- deploy_file: copies one file/symlink, backing up only the conflicting
  destination (if any). Idempotent via paths_equivalent() so re-runs
  with no changes produce no output and no spurious backups.
- deploy_dir: walks the repo's tree for that dir and deploys each leaf
  via deploy_file. Files in $HOME the repo doesn't know about are left
  untouched. Subdirs are mkdir'd as needed.

Also: track the personal fonts at .local/share/fonts/ so they redeploy
on every install, and run fc-cache -f at the end so apps see them
without a logout/login.

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

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