M5: stamps without an appearance draw the stamp they name (ISO 32000-2 Table 184)

A rubber stamp from another program with no normal appearance was not
drawn at all. The display copy now gives it one:

- the fourteen standard names of Table 184 draw that stamp with
  PdfCraft's stamp drawing (eight from the palette, six with labels of
  their own), fitted to /Rect, in /C or the stamp's colour;
- no /Name draws Draft, the table's default, unless /IT says the stamp
  is an image or a snapshot;
- any other name (names match byte for byte, §7.3.5) draws a thin grey
  frame with both diagonals, so the stamp stays visible and selectable.

The drawing is display-only, like the other comments without an
appearance: Save writes the file as it was, and build() still declines
these stamps, so restyling and flattening never write it into the file.
A stamp with its own /AP /N is never touched. stamp() now takes the
label and the sign-here flag instead of a StampKind.

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

crates/annot/src/lib.rs

@@ -1586,7 +1586,14 @@ pub fn with_missing_appearances_cached(doc: &Document, cache: &mut AppearanceCac
                 apply_appearance(copy.get_or_insert_with(|| doc.clone()), r, &d, stream).ok();
                 continue;
             }
-            let Some(stream) = build_appearance(doc, &d) else { continue };
+            // Another program's stamp gets a drawing for display only (ISO 32000-2 Table 184). Its
+            // `/AP`, if any, has no `/N` to keep here.
+            let other_stamp = || {
+                let mut bare = d.clone();
+                bare.remove(b"AP");
+                appearance::display_stamp(&bare)
+            };
+            let Some(stream) = build_appearance(doc, &d).or_else(other_stamp) else { continue };
             cache.insert(r, &stream);
             // Fails only when `r` isn't a dictionary, which the build just ruled out.
             apply_appearance(copy.get_or_insert_with(|| doc.clone()), r, &d, stream).ok();