generate: normalize worlds into collections of features

Zones, paths and points are about to hold equal rank on the wire, and
that cannot happen while the generator itself thinks in the MapGenie
shape -- pins under groups and categories, regions standing apart as
zones, every pass written twice because of it. The catalog now holds
one model instead: collections of features, where each category is a
point collection and a world's regions fold into one implicit area
collection. Every pass over a map -- conventions, icons, links,
sharding, surfaces, merging, the merge gate -- reads that model.

The wire does not move an inch yet. The v2 groups and zones are
reconstructed from the collections at the moment a payload is written,
and the emission-only structs stay behind to spell them; regrouping by
first-seen group id reproduces the original tree because categories
within a group were contiguous in every capture and every pass keeps
them so. A rebuilt Westminster bundle lands under the same stamp, and
its manifest and world payloads compare byte for byte against the
pre-refactor build -- which is the proof that the model holds
everything the wire says, and the licence for the wire to move later.

The pin count is no longer a field that could drift: the manifest
counts the point features it actually has, and the merge gate holds
that count to the sum of the map's own ledger.

commit

37327fa0fa66c5c50b789baac693355d8715dd0f

parent

6467a1c789b7e72ffc8bda662d024f980234b165

author

Dami <dami.etoile@gmail.com>

kind

refactor risk score 2.0 of 3, Jev's estimate