More underbrush-like terrain which can be foraged

### Is your feature request related to a problem? Please describe.

We only have the underbrush to forage yet we have different biomes. It would be amazing if there was the possibility of having more foragable terrain, each with their own loot table.
There are already a couple of good usecases:

1. Pile of sand, found on the shore and close to rivers. Can yield crabs and river debris and the like.
2. Tangled mound of dirt. Found in, who'd have guessed, swamps. Yields loot that is more appropriate to swamps. Different bird eggs, more reptile eggs perhaps and different plants.
3. Fungal underbrush. Found in mycus territory. Can yield mycus matter and maybe even one of those elusive berries?
4. Tree stump or fallen log with mushrooms. Yields mainly/only shrooms.

### Solution you would like.

I believe the best way to accomplish that is using `examine_action`. Like this (concept stolen from effects on condition):

```json
"examine_action": { "type": "forage", "itemgroups": { "spring": "foraging_beach_spring" } }
```

The tricky part is that our current underbrush turns into `t_underbrush_harvested_spring` when harvested in spring and turns back into `t_underbrush` when the season changes, so this is unfortunately not a good first issue.

### Describe alternatives you have considered.

_No response_

### Additional context

_No response_

Start with the existing examine_action forage behavior and the seasonal transitions between t_underbrush and t_underbrush_harvested_spring. Trace how terrain-specific itemgroups are selected and how harvested terrain regrows across seasons. Done means adding the proposed terrain variants and loot tables while preserving the current harvest and seasonal behavior.

Written by the indexing model from the issue text.