Consolidate the jsonpull comments

Comments were 25% of the added lines, and the ownership model was spelled
out in five places. Collect it into one block at the top of jsonpull.h and
point at it from the rest, cutting the ratio to 14% and the total by about
200 lines.

Removed the duplicate explanations of the deleter dispatch, of what detach
does to the back-pointers, and of "json_read returns intermediate
containers, do not free them". Trimmed the comments that argued for a
choice rather than described the code -- the reserve(2) / reserve(4)
rationales, the string-buffer copy, the pmtiles check ordering -- to a line
each, and shortened the test preambles, keeping the parts that say why a
test is shaped the way it is.

No code changes; the test suite is unchanged in both configurations.

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

dirtiles.cpp

@@ -261,12 +261,9 @@ 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.)
+			// Skip, not just warn: a metadata.json from another tool may
+			// have a numeric minzoom or a nested object, and string()
+			// asserts on the type.
 			if (e.key->type != JSON_STRING || e.value->type != JSON_STRING) {
 				fprintf(stderr, "%s: non-string in metadata\n", name.c_str());
 				continue;