stocks: make the overlay's collapse hotkey a toggle

[Alistair-Afton]

Fixes #5773

Summary

• The overlay's collapse all hotkey (Ctrl-X) is now a toggle: when every category is collapsed it expands the list again instead of doing nothing.
• The hotkey label flips between collapse all and expand all so it always describes what the key will do.
• The separate expand all hotkey (Ctrl-Z) is unchanged, so expanding is still a single press when only some categories are collapsed.

Testing

• Exercised toggle_all/all_collapsed in-game against mocked stocks state: collapse -> expand -> collapse cycle flips flags and recomputes i_height correctly each way, and the scroll reset from stocks: fix blank item list after collapsing while scrolled down #5905 still applies on the collapse half.

Note: this branch is based on stocks-scroll-bounds (#5905) since it uses the module-level functions from that change. The diff will shrink to just this change once #5905 merges.

[SilasD]

The new functions all_collapsed and toggle_all have the same issue I raised in PR #5905 . to reiterate:

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

these functions directly manipulate or read 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.

this is true even of all_collapsed, which only reads the UI state. even walking a vector of boolean might be problematic if the stocks window has never been opened; I am not at all convinced that the vector would have valid contents instead of garbage.

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.

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

[Alistair-Afton]

Done both here as well: collapse_all, expand_all, all_collapsed, toggle_all, and remove_empty are all local again, and each is guarded by stocks.open (all_collapsed returns true when closed, matching the empty-vector result).

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

• the toggle hotkey collapses all sections, then expands them again when everything is collapsed
• collapse resets scroll_position_item
• expand-all restores all sections
• remove-empties keeps scroll_position_type in bounds
• all 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).