schemagen: reject duplicate unknown keys in non-strict objects

Track unknown keys in a per-object unordered_set so that permissive
objects (additionalProperties absent/true) still reject duplicate keys,
matching the README guarantee.

- Add std::unordered_set<std::string> to Frame.
- Insert unknown keys in cbKeyData and reject duplicates before skipping.
- Add a permissive "loose" subobject to example.schema.json.
- Test single unknown key accepted and duplicate unknown/known keys rejected.

contrib/schemagen/weaseljson_schemagen.py

@@ -566,6 +566,7 @@ class Emitter:
 #include <optional>
 #include <string>
 #include <string_view>
+#include <unordered_set>
 #include <vector>
 
 #include "weaseljson.h"
@@ -931,6 +932,7 @@ private:
     void *dest;
     int field = -1;              // object: selected field (-1 want key, -2 skip)
     std::vector<uint64_t> seen;  // populated field bitset (object frames)
+    std::unordered_set<std::string> unknown;  // unknown keys seen in non-strict objects
   }};
   static constexpr int kWantKey = -1;
   static constexpr int kSkip = -2;
@@ -1014,12 +1016,16 @@ private:
     scratch_.append(buf, len);
     if (!done) return;
     int idx = matchKey(f.kind, scratch_);
-    scratch_.clear();
     if (idx < 0) {{
       if (isStrict(f.kind)) {{ reject(); return; }}
+      if (!f.unknown.insert(scratch_).second) {{
+        reject(); return;  // duplicate unknown key
+      }}
+      scratch_.clear();
       f.field = kSkip;
       return;
     }}
+    scratch_.clear();
     if (f.seen[idx >> 6] & (1ull << (idx & 63))) {{
       reject(); return;  // duplicate key
     }}