M5: the placeholder for an unknown stamp name stays grey whatever its /C

An AP-less stamp whose /Name has no drawing got its placeholder frame in
the annotation's /C colour (/C [1 0 0] drew a red frame), while the doc
comment and parity note promise a neutral grey one. The frame now always
strokes PLACEHOLDER_GREY; the fourteen Table 184 names keep /C.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0175FWj4xFhMhSH3UpMU8DSC

crates/annot/src/appearance.rs

@@ -843,9 +843,10 @@ fn standard_stamp(name: &[u8]) -> Option<(&'static str, Rgb)> {
 const PLACEHOLDER_GREY: Rgb = [0.45, 0.45, 0.45];
 
 /// The display appearance of a rubber stamp another program wrote without one: the stamp its
-/// `/Name` names (Draft when there is none, Table 184's default), or for any other name a thin
-/// grey frame with both diagonals, so the stamp stays visible and can be selected. The name is
-/// never drawn as text.
+/// `/Name` names (Draft when there is none, Table 184's default) in its `/C` colour or the
+/// stamp's own, or for any other name a thin grey frame with both diagonals, so the stamp stays
+/// visible and can be selected. The frame is grey whatever the `/C`: it stands in for a stamp
+/// that cannot be drawn, so it takes none of that stamp's colour. The name is never drawn as text.
 ///
 /// For display only ([`crate::with_missing_appearances`]): [`build`] declines these stamps, so
 /// edits (restyling, flattening) never write this drawing into the file. `None` for any other
@@ -884,7 +885,8 @@ pub(crate) fn display_stamp(d: &Dict) -> Option<Stream> {
     }
     c.push_str(&format!(
         "{}1 w\n{} {} {} {} re\n{} {} m {} {} l\n{} {} m {} {} l\nS\n",
-        rg_stroke(stroke.unwrap_or(PLACEHOLDER_GREY)),
+        // Grey whatever the `/C`: the placeholder is not the stamp, so it takes none of its colour.
+        rg_stroke(PLACEHOLDER_GREY),
         n(a0),
         n(b0),
         n(a1 - a0),

crates/annot/src/tests.rs

@@ -1064,6 +1064,9 @@ fn a_stamp_without_an_appearance_draws_the_stamp_it_names() {
     }
     // `/C` colours the stamp, as it does PdfCraft's own.
     assert!(drawn(&stamp_dict("/Name /Final /C [1 0 1]")).contains("1 0 1 RG"));
+    // The placeholder stays grey whatever the `/C`: it is not the stamp, so it takes none of its colour.
+    let placeholder = drawn(&stamp_dict("/Name /MyCustomStamp /C [1 0 0]"));
+    assert!(placeholder.contains("0.45 0.45 0.45 RG") && !placeholder.contains("1 0 0 RG"), "{placeholder}");
     // A stamp's own artwork is never replaced, and the drawing is for display only: edits
     // (restyling, flattening) still find nothing they may write.
     assert!(appearance::display_stamp(&stamp_dict("/Name /Draft /AP << /N 9 0 R >>")).is_none());