Migrate jsonpull to unique_ptr ownership

Replaces the shared_ptr-based json_object_ptr with a unique_ptr that
has a stateless custom deleter dispatching on json_object::type before
calling the right subclass destructor. Eliminates per-node atomic
reference-counting and the control-block allocation that shared_ptr
required for every node in the tree.

API now distinguishes owning and borrowing pointers explicitly:
- json_read / json_read_separators / json_hash_get return raw
  json_object * (borrowed from the parser-owned tree).
- json_read_tree / json_disconnect return json_object_ptr (caller
  takes ownership; back-pointers are cleared so the subtree can
  outlive the parser).
- json_free / json_context / json_stringify take raw pointers.
- The parser's container_stack holds raw pointers; jp->root keeps
  unique_ptr ownership of the most recent top-level value.

Internally, take_from_owner moves the unique_ptr out of whichever
parent vector / hash entry / parser root owned it, which both
json_free and json_disconnect rely on.

In the streaming parsers (parse_feature, parse_layers, the
geojson-loop callback), we are careful to free `j` only after we
have processed a complete Feature: json_read returns each token
as the tree is being built up, and freeing an intermediate node
would splice it out of the surrounding hash and corrupt the
in-progress feature.

Benchmark (tl_2022_us_county.json, -z0 --extend-zooms-if-still-dropping,
median of 5 runs on macOS arm64): 8.5s, vs 10.6s with shared_ptr
and 8.7s on the pre-refactor C baseline.

Co-authored-by: Cursor <cursoragent@cursor.com>

overzoom.cpp

@@ -264,7 +264,7 @@ int main(int argc, char **argv) {
 			its.push_back(std::move(t));
 		}
 
-		out = overzoom(its, nz, nx, ny, detail, buffer, keep, exclude, exclude_prefix, do_compress, NULL, demultiply, json_filter, preserve_input_order, attribute_accum, unidecode_data, simplification, tiny_polygon_size, std::vector<mvt_layer>(), "", "", SIZE_MAX, std::vector<clipbbox>(), deduplicate_by_id);
+		out = overzoom(its, nz, nx, ny, detail, buffer, keep, exclude, exclude_prefix, do_compress, NULL, demultiply, json_filter.get(), preserve_input_order, attribute_accum, unidecode_data, simplification, tiny_polygon_size, std::vector<mvt_layer>(), "", "", SIZE_MAX, std::vector<clipbbox>(), deduplicate_by_id);
 	}
 
 	FILE *f = fopen(outfile, "wb");