Villager secondary work blocks

Add blocks to a vanilla profession's secondary work sites with the block tag horizon:villager_secondary_poi/<profession>. For example, data/horizon/tags/block/villager_secondary_poi/farmer.json:

{"values": ["minecraft:soul_sand"]}

Entries add to the profession's defaults. Farmers retain farmland, including blocks using the farmland template. Other vanilla professions can use their own names, such as fisherman. Mod professions await profession registration.

Tags use the normal world-open and /reload tag resolution. Successful reloads replace membership; failed reloads retain the previous world data. Removing all entries restores vanilla behavior.

The server sensor keeps its existing cadence and inclusive scan of four blocks in x and z and two in y. Professions with empty secondary sets skip the scan. Results populate secondary job-site brain memory. The shared work activity can stroll toward a randomly selected secondary site while near its primary job site. Farming still requires a farmer and retains its existing crop and farmland checks.