Issue #92: XMP: merge into an existing sidecar instead of replacing it

Saving (Cmd+S or auto-write) now merges LightCraft's properties into an
existing sidecar (lightcraft_meta::merge_xmp): owned properties (rating,
label, dc/IPTC/rights fields, lc:*) are replaced or removed, capture time
and GPS only when LightCraft has a value; everything else (crs:,
xmpMM:History, unknown namespaces, packet wrapper) is kept byte for byte.
The result is re-parsed and checked before it is written (temp + fsync +
rename). A sidecar that isn't valid XMP is copied to .xmp.bak-<time>
(never overwriting) before it is replaced; one that can't be read at all
is left alone and the save fails.

Shared stems: when several catalogued files map to one stem sidecar
(IMG.CR3 + IMG.JPG), it belongs to the raw (else the first by name) and
the others write and read <file>.xmp.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/query.rs

@@ -113,7 +113,7 @@ pub fn specs() -> Vec<CommandSpec> {
                 .map(|r| json!({"group": r.group, "tag": r.tag, "name": r.name, "value": r.value}))
                 .collect();
             // XMP: the sidecar if there is one, else the file's own packet
-            let packet = crate::sidecar::read_packet(path, ph.kind, s.xmp.naming).map(|(x, _)| x).or_else(|| lightcraft_meta::embedded(&bytes).xmp);
+            let packet = crate::sidecar::read_packet(path, ph.kind, s.sidecar_naming(ph.id)).map(|(x, _)| x).or_else(|| lightcraft_meta::embedded(&bytes).xmp);
             let xmp: Vec<Value> = packet
                 .and_then(|x| lightcraft_meta::parse_xmp(&x).ok())
                 .map(|d| d.properties.into_iter().filter(|(k, _)| !k.starts_with("lc:")).map(|(k, v)| json!({"name": k, "value": v.join("; ")})).collect())