Farms & Greenhouses Anywhere
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Adds four new buildings next to the vanilla ones in the Food production
toolbar category:

  * Farm (Any Terrain)
  * Irrigated Farm (Any Terrain)
  * Greenhouse (Any Terrain)
  * Greenhouse II (Any Terrain)

Each is an exact copy of the matching vanilla building - same layout, same
yield, same recipes, same upgrade tier data - except it can be built on
ANY terrain (rock, gravel, sand, ...), not just fertile ground.

Cost: +250 Dirt on top of the vanilla building cost. This is a one-time
construction cost only. It has nothing to do with harvesting - once
built, the building farms and harvests exactly like the original,
everywhere, since the game tracks fertility internally per-building and
never rechecks the terrain at runtime.

Research: each variant has its own research node, placed directly behind
the matching vanilla node in the research tree, with the exact same
research cost as the original.

Building collision (can't build on top of other buildings, conveyors,
pipes, ramps) is completely untouched.

IMPORTANT: Because this mod adds new building types, a save that has any
of these buildings placed in it depends on the mod going forward. Do not
remove the mod from a save that still has any-terrain farms built - the
game won't know what to do with the missing building type. Removing the
mod is safe as long as none of the four new buildings have been built.

Technical detail: a single Harmony prefix on
FarmFertileGroundValidator.CanAdd lets the four new building IDs skip the
soil-fertility construction check; everything else (production, recipes,
upgrades) runs on the game's own FarmProto/Farm code, unmodified.
