M2.1: Only inspect the Fit One Page DOCX part by name (#6433)

Check the fixed custom XML part name before parsing, so unrelated custom XML
stores are not read twice just to look for the fit budget.

crates/docx/src/fit.rs

@@ -18,6 +18,11 @@ pub(crate) const NS: &str = "urn:wordcraft:fit";
 /// Marker attribute the parser keeps (namespace declarations are stripped from `El`).
 const KIND: &str = "wordcraft-fit-v1";
 
+/// Whether `target` (a relationship target) names our part.
+pub(crate) fn is_part(target: &str) -> bool {
+    target.trim_start_matches('/').eq_ignore_ascii_case(PART) || target.to_ascii_lowercase().ends_with(&format!("/{PART}"))
+}
+
 /// Whether `root` is our store (and not a user's custom XML part with a coincidental name).
 pub(crate) fn is_state(root: &El) -> bool {
     root.local() == ROOT && root.attr("kind") == Some(KIND)

crates/docx/src/read/embed.rs

@@ -120,6 +120,9 @@ impl Reader<'_> {
                 if ![&rel.kind, &rel.target].iter().all(|s| manifest_field_ok(s)) {
                     continue;
                 }
+                if crate::fit::is_part(&rel.target) {
+                    continue;
+                }
                 let root = self.xml(&rel.target).ok().flatten();
                 if root.as_ref().is_some_and(crate::fit::is_state) {
                     continue;
@@ -146,7 +149,7 @@ impl Reader<'_> {
     /// Read the Fit One Page budget from our custom XML store, if the file has one.
     pub(super) fn read_fit_state(&mut self, document: &Rels) {
         for rel in document.list.iter().filter(|r| !r.external && rel_is(&r.kind, rt::CUSTOM_XML)).take(MAX_EMBEDDED_RELS) {
-            if !manifest_field_ok(&rel.target) {
+            if !manifest_field_ok(&rel.target) || !crate::fit::is_part(&rel.target) {
                 continue;
             }
             let Some(root) = self.xml(&rel.target).ok().flatten() else { continue };