infocmp -C: translate %{K}%+%c and escape %+ arguments standalone

Add the %{K}%+%c -> %+<char K> translation (add-constant, emit-as-char), mirroring
the existing %'X'%+%c form. The %+ argument is a single character escaped on its own
(caret for control bytes), independent of the surrounding value's caret-vs-octal
length test -- so adm20's cm renders \E=%i%r%+^_%+^_ (caret), matching ncurses, while
a literal control byte like a980's al \016 still follows the length heuristic (octal).

Termcap (-C) content 66.3% -> 66.4%. terminfo -1 100%; tests green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CbiPgWC3PHKmrTNvZzXcWQ

Author: claude

crates/ncurses-tools/src/bin/infocmp.rs

@@ -379,9 +379,39 @@ fn try_translate(v: &[u8], islong: bool) -> Option<String> {
                     r_pending = false;
                 }
                 out.push_str("%+");
-                tc_escape_byte(&mut out, v[i + 2], islong, v.get(i + 3).copied());
+                // The `%+` argument is a standalone char, escaped on its own (caret for control),
+                // independent of the surrounding value's caret-vs-octal length.
+                tc_escape_byte(&mut out, v[i + 2], false, None);
                 i += 8;
             }
+            // `%{K}%+%c` (add a constant, emit as char) -> termcap `%+<char K>`, like the `%'X'`
+            // form above (e.g. `%p1%{24}%+%c` -> `%+^X`).
+            Some(b'{') => {
+                let mut j = i + 2;
+                while j < v.len() && v[j].is_ascii_digit() {
+                    j += 1;
+                }
+                if j == i + 2
+                    || v.get(j) != Some(&b'}')
+                    || v.get(j + 1) != Some(&b'%')
+                    || v.get(j + 2) != Some(&b'+')
+                    || v.get(j + 3) != Some(&b'%')
+                    || v.get(j + 4) != Some(&b'c')
+                {
+                    return None;
+                }
+                let k: u32 = std::str::from_utf8(&v[i + 2..j]).ok()?.parse().ok()?;
+                if k > 0xff {
+                    return None;
+                }
+                if r_pending {
+                    out.push_str("%r");
+                    r_pending = false;
+                }
+                out.push_str("%+");
+                tc_escape_byte(&mut out, k as u8, false, None);
+                i = j + 5;
+            }
             // A `%` before a byte that is not a real parameter operator is copied through verbatim,
             // exactly as _nc_infotocap does -- it only fails (marking the cap `..`) on genuine
             // parameter/conditional/stack ops. This covers exotic printer codes (`%z`, `%y`, `%}`,