stocks: fix blank item list after collapsing while scrolled down

[Alistair-Afton]

Summary

collapse_all shrank i_height but left scroll_position_item stale, so collapsing a list while scrolled down left the viewport past the end of the list and rendered a blank pane (fixes #5772).

Applied the same scroll clamp to remove_empty, which had the identical hazard on the type list, and fixed a leaked global (num_items in expand_all was missing local).

Testing

• Live fort: collapse resets scroll to 0; flags continue toggling correctly
• ci/lint.py clean

[SilasD]

I am very uncomfortable making collapse_all, expand_all, and remove_empty global.

these functions directly manipulate the UI; the UI is generally fragile and tends to leave dangling pointers when a viewsheet is closed.

making these functions global means that any other script can call them at any time. if the stocks viewsheet is not open, I think there is a significant risk of crashing the game.

to emphasize: these functions should (must) not be called if the stocks interface is not open.

I would either like these changed back to local or have a safety-test added, something like
if stocks.open == false then return; end
at the start of each function.

or both. both would be good.

---

Other than that issue, the code generally looks fine, though I didn't test it.

[Alistair-Afton]

Done both: the functions are local again, and each now starts with if not stocks.open then return end so a stray activation as the page closes can't touch the interface state.

Also added test/plugins/stocks.lua, which drives the overlay hotkeys through the widget's real onInput path in a running game and verifies:

• collapse resets scroll_position_item
• expand restores all sections
• remove-empties keeps scroll_position_type in bounds
• all three hotkeys are no-ops while the stocks page is closed (exercises the stocks.open guard directly)

Verified in-game: 4/4 tests pass, full suite green (271/271).