Ability to exclude plots from taxation in realty

This should be on a tags basis, let me grab what I was thinking

Code context — realty-paper/…/listener/PropertyTaxListener only supports exempting a whole owner (TaxSettings.exemptUuids) or a flat per-owner free-plot count (exemptPlotThreshold); there is no per-region/per-plot exemption — tax is computed per owner over regionsByOwner.

Fix: add an exempt-regions concept (an exempt-regions setting and/or a per-region flag) and drop exempt regions from the owner's plot set before policy.taxForOwner(...) in the per-owner loop.

State corrected Planned → Done: realty 154beb4 ("Configurable per-property tax by region tag + formula") is on main (released, v1.3.2). Plots are excluded from taxation by configuring the region tag's tax formula (exemption is in use in prod). If you intended a dedicated per-plot exempt flag rather than the tag-config route, reopen and I'll scope that.