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

pmtiles_file.cpp

@@ -416,11 +416,7 @@ sqlite3 *pmtilesmeta2tmp(const char *fname, const char *pmtiles_map) {
 	state.json_write_hash();
 
 	for (const auto &e : o->entries()) {
-		// Establish that the key really is a string before reading it as
-		// one, rather than after: string() asserts on the type, so the
-		// check has to come first to be the thing that catches a bad key.
-		// (The parser rejects non-string hash keys, so this is belt and
-		// braces, but the ordering is what makes it meaningful.)
+		// Check before reading, since string() asserts on the type.
 		if (e.key->type != JSON_STRING) {
 			fprintf(stderr, "%s: non-string key in metadata\n", fname);
 			continue;