Fix stack overflow in no-musttail fallback for direct continuations

The no-musttail trampoline (063872e) only rerouted keepGoing dispatch.
Continuations that tail-call each other directly still recursed on the C
stack when MUSTTAIL is a no-op, so deep input overflowed it -- nested
arrays via n_value <-> n_array2, and runs of string escapes via
n_string2 -> n_string2 (the latter isn't even bounded by stackSize).

Add a TAILCALL(fn) macro: a direct musttail tail call when available,
otherwise a bounce to the parse() trampoline (which re-dispatches
continuations[top()] == fn, valid because fn's symbol is already on top
of the stack at every site). Route the 10 direct continuation tail calls
through it. The musttail build is unchanged; the fallback no longer
recurses without bound.

src/parser3.h

@@ -161,6 +161,18 @@ struct Parser3 {
 #endif
 };
 
+// Transfer control to continuation `fn`, whose Symbol must already be on top of
+// the stack. With musttail this is a direct tail call (fast path). Without it,
+// a direct call would recurse on the C stack (e.g. n_value -> n_array2 ->
+// n_value for nested arrays, or n_string2 -> n_string2 for runs of escapes) and
+// overflow on deep input; instead we bounce to the parse() trampoline, which
+// re-dispatches continuations[top()] == fn without growing the stack.
+#ifdef HAS_MUSTTAIL
+#define TAILCALL(fn) MUSTTAIL return fn(self, buf, bufEnd)
+#else
+#define TAILCALL(fn) return Parser3::keepGoing(self, buf, bufEnd)
+#endif
+
 inline PRESERVE_NONE ContinuationStatus skipWhitespace(char *&buf,
                                                        char *bufEnd) {
   constexpr int kStride = 4;
@@ -306,7 +318,7 @@ inline PRESERVE_NONE ContinuationStatus n_value(Parser3 *self, char *buf,
     if (buf == bufEnd) {
       return WeaselJson_AGAIN;
     }
-    MUSTTAIL return n_object2(self, buf, bufEnd);
+    TAILCALL(n_object2);
   case '[':
     self->callbacks->on_begin_array(self->userdata);
     ++buf;
@@ -315,7 +327,7 @@ inline PRESERVE_NONE ContinuationStatus n_value(Parser3 *self, char *buf,
     if (buf == bufEnd) {
       return WeaselJson_AGAIN;
     }
-    MUSTTAIL return n_array2(self, buf, bufEnd);
+    TAILCALL(n_array2);
   case '"':
     ++buf;
     self->dataBegin = self->writeBuf = buf;
@@ -325,7 +337,7 @@ inline PRESERVE_NONE ContinuationStatus n_value(Parser3 *self, char *buf,
     if (buf == bufEnd) {
       return WeaselJson_AGAIN;
     }
-    MUSTTAIL return n_string2(self, buf, bufEnd);
+    TAILCALL(n_string2);
   case '0':
   case '1':
   case '2':
@@ -344,7 +356,7 @@ inline PRESERVE_NONE ContinuationStatus n_value(Parser3 *self, char *buf,
     if (buf == bufEnd) {
       return WeaselJson_AGAIN;
     }
-    MUSTTAIL return n_number(self, buf, bufEnd);
+    TAILCALL(n_number);
   case 't':
     ++buf;
     self->pop();
@@ -432,7 +444,7 @@ inline PRESERVE_NONE ContinuationStatus n_object2(Parser3 *self, char *buf,
     if (buf == bufEnd) {
       return WeaselJson_AGAIN;
     }
-    MUSTTAIL return n_string2(self, buf, bufEnd);
+    TAILCALL(n_string2);
   default:
     [[unlikely]] return WeaselJson_REJECT;
   }
@@ -478,7 +490,7 @@ inline PRESERVE_NONE ContinuationStatus n_object3(Parser3 *self, char *buf,
     if (buf == bufEnd) {
       return WeaselJson_AGAIN;
     }
-    MUSTTAIL return n_string2(self, buf, bufEnd);
+    TAILCALL(n_string2);
   default:
     [[unlikely]] return WeaselJson_REJECT;
   }
@@ -506,7 +518,7 @@ inline PRESERVE_NONE ContinuationStatus n_array2(Parser3 *self, char *buf,
     if (auto s = self->push({N_VALUE, N_ARRAY3})) {
       return s;
     }
-    MUSTTAIL return n_value(self, buf, bufEnd);
+    TAILCALL(n_value);
   }
 }
 
@@ -536,7 +548,7 @@ inline PRESERVE_NONE ContinuationStatus n_array3(Parser3 *self, char *buf,
     if (buf == bufEnd) {
       return WeaselJson_AGAIN;
     }
-    MUSTTAIL return n_value(self, buf, bufEnd);
+    TAILCALL(n_value);
   default:
     [[unlikely]] return WeaselJson_REJECT;
   }
@@ -561,7 +573,7 @@ inline PRESERVE_NONE ContinuationStatus n_string(Parser3 *self, char *buf,
   if (buf == bufEnd) {
     return WeaselJson_AGAIN;
   }
-  MUSTTAIL return n_string2(self, buf, bufEnd);
+  TAILCALL(n_string2);
 }
 
 inline int32_t read4_hex(const char *buf) {
@@ -664,7 +676,7 @@ inline PRESERVE_NONE ContinuationStatus n_string2(Parser3 *self, char *buf,
         self->flushString(false, buf);
         return WeaselJson_AGAIN;
       }
-      MUSTTAIL return n_string2(self, buf, bufEnd);
+      TAILCALL(n_string2);
     }
   default:
     [[unlikely]] return WeaselJson_REJECT;
@@ -1072,4 +1084,6 @@ inline PRESERVE_NONE ContinuationStatus Parser3::keepGoing(Parser3 *self,
 #endif
 }
 
+#undef TAILCALL
+
 } // namespace parser3