Optimize alt linking to avoid redundant operations (fixes #236)

[slowsage]

What does this PR do?

This change reduces unnecessary filesystem operations and preserves user‑created symlinks when no alternate is tracked. Stale links are still removed (#236). Added two tests. #65 still passes.

What issues does this PR fix or reference?

#236

Previous Behavior

#236

ln -s /nix/store/abc123/bashrc ~/.bashrc              # User's symlink
echo "content" > ~/.config/yadm/alt/.bashrc##os.Darwin  # (assume not on Darwin)
yadm add ~/.config/yadm/alt/.bashrc##os.Darwin         # Track it
yadm alt # Result: ~/.bashrc REMOVED (even with score=0)

Touch only when necessary:

yadm alt;yadm alt  # (mtime changes on second alt)

New Behavior

#236

ln -s /nix/store/abc123/bashrc ~/.bashrc              # User's symlink
echo "content" > ~/.config/yadm/alt/.bashrc##os.Darwin  # (assume not on Darwin))
yadm add ~/.config/yadm/alt/.bashrc##os.Darwin         # Track it
yadm alt # Result: ~/.bashrc unchanged (score=0 for Darwin, skipped)

Touch only when necessary

yadm alt;yadm alt # mtime unchanged

Have tests been written for this change?

Yes

Have these commits been signed with GnuPG?

Yes

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