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

read_json.cpp

@@ -321,11 +321,7 @@ std::vector<mvt_layer> parse_layers(FILE *fp, int z, unsigned x, unsigned y, int
 			break;
 		}
 
-		// json_read returns each parser token in sequence, including
-		// intermediate (still-incomplete) container nodes. Freeing those
-		// here would splice them out of the feature hash being built
-		// up, so only free `j` once we have processed a complete
-		// Feature (or `jp->root` when the stream ends).
+		// Only complete Features are freed; see plugin.cpp::parse_feature.
 		json_object *type = json_hash_get(j, "type");
 		if (type == nullptr || type->type != JSON_STRING) {
 			continue;