feat: improve cache of copy targets

For templates and symlinks, it is easy to see if the target has
changed (e.g., symlink is destroyed, or the rendered template differs
to the target). This is not as easy to do with copy target; if the
source and target differ, it is unclear whether the source was modified
and the target is stale, or the target was modified outside of
dotter (or both).

This changes the caching so that it stores a hash (using `xxhash`) of
both source and target. On redeployment, the hashes can be compared and
dotter can determine whether it is safe to override the target file.

Assisted-by: Claude Code:claude-sonnet-4-6
Signed-off-by: JP-Ellis <josh@jpellis.me>

Author: JP-Ellis

Cargo.lock

@@ -22,6 +22,7 @@ hostname = "0.3.*"
 log = "0.4.*"
 maplit = "1.*"
 evalexpr = "13"
+xxhash-rust = { version = "0.8", features = ["xxh3"] }
 serde = { version = "1.*", features = ["derive"] }
 shellexpand = "2.*"
 simplelog = "0.12.*"