Skip non-string metadata.json entries instead of reading them as strings

dirmeta2tmp() warned about a metadata entry that was not a string/string
pair and then read it as a string anyway. Under the new type-tagged
accessors that trips the assert in json_object::string(); before them it
reinterpreted the node's storage as a char pointer, which segfaulted for
most values. Either way, tippecanoe-decode and tile-join could not read a
directory tileset whose metadata.json had a numeric minzoom or a nested
object, which is common in metadata.json files written by other tools.

Add the missing continue, and cover it in raw-tiles-test.

pmtilesmeta2tmp() handles the same case correctly but read the key with
string() before its own JSON_STRING check, so the assert would have fired
ahead of the check meant to catch a bad key. Hoist the check above the
read. The parser rejects non-string hash keys, so this is unreachable in
practice; the ordering is what makes the check meaningful.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017KNxyHKasyWrWcvre2yK4r

dirtiles.cpp

@@ -261,8 +261,15 @@ sqlite3 *dirmeta2tmp(const char *fname) {
 		}
 
 		for (const auto &e : o->entries()) {
+			// Skip, rather than just warn about, anything that isn't a
+			// string/string pair: reading a non-string through string()
+			// would assert in a debug build and misinterpret the node's
+			// storage in a release build. (A metadata.json written by
+			// something other than tippecanoe may well have numeric
+			// minzoom/maxzoom or a nested "json" object.)
 			if (e.key->type != JSON_STRING || e.value->type != JSON_STRING) {
 				fprintf(stderr, "%s: non-string in metadata\n", name.c_str());
+				continue;
 			}
 
 			char *sql = sqlite3_mprintf("INSERT INTO metadata (name, value) VALUES (%Q, %Q);", e.key->string().c_str(), e.value->string().c_str());