Parser3::reset: clear inKey and other per-parse transient state

WeaselJsonParser_reset is documented to restore the parser to its
newly-created state, but reset() only rewound the symbol stack.  The
inKey flag and transient DFA/codepoint state from the previous parse
leaked into the next parse, so a top-level string after a mid-key
reset was delivered via on_key_data instead of on_string_data.

Reset inKey to false and clear utf8Codepoint, utf16Surrogate,
minCodepoint, numDfa, and strDfa so the next document starts fresh.

Add a test that reproduces the reported misrouting.

src/parser3.h

@@ -138,6 +138,12 @@ struct Parser3 {
   void reset() {
     stackPtr = stack();
     std::ignore = push({N_VALUE, N_WHITESPACE, T_EOF});
+    inKey = false;
+    utf8Codepoint = 0;
+    utf16Surrogate = 0;
+    minCodepoint = 0;
+    numDfa.reset();
+    strDfa.reset();
   }
 
   // Used for flushing pending data with on_*_data callbacks

src/test.cpp

@@ -223,6 +223,47 @@ TEST_CASE("create rejects too-small stack") {
 
 TEST_CASE("streaming") { testStreaming(json); }
 
+TEST_CASE("reset clears inKey and transient state") {
+  struct State {
+    std::string stringData;
+    std::string keyData;
+  } state;
+  auto c = noopCallbacks();
+  c.on_string_data = +[](void *p, const char *buf, int len, int /*done*/) {
+    ((State *)p)->stringData.append(buf, len);
+  };
+  c.on_key_data = +[](void *p, const char *buf, int len, int /*done*/) {
+    ((State *)p)->keyData.append(buf, len);
+  };
+
+  auto *parser = WeaselJsonParser_create(1024, &c, &state, 0);
+  REQUIRE(parser != nullptr);
+
+  {
+    std::string chunk = "{\"ab";
+    REQUIRE(WeaselJsonParser_parse(parser, chunk.data(), chunk.size()) ==
+            WeaselJson_AGAIN);
+  }
+
+  // Reset mid-key: the next top-level string must be delivered as a string,
+  // not appended to the aborted key.
+  WeaselJsonParser_reset(parser);
+  state.stringData.clear();
+  state.keyData.clear();
+
+  {
+    std::string chunk = "\"hello\"";
+    REQUIRE(WeaselJsonParser_parse(parser, chunk.data(), chunk.size()) ==
+            WeaselJson_AGAIN);
+  }
+  REQUIRE(WeaselJsonParser_parse(parser, nullptr, 0) == WeaselJson_OK);
+
+  CHECK(state.stringData == "hello");
+  CHECK(state.keyData.empty());
+
+  WeaselJsonParser_destroy(parser);
+}
+
 void doTestUnescapingUtf8(std::string const &escaped,
                           std::string const &expected, int stride, int flags) {
   CAPTURE(escaped);