sort: clickable stress icons in the top bar

[Alistair-Afton]

Summary

• Adds an invisible hotspot overlay widget (sort.stress_icons) over the seven stress/happiness icons next to the "Pop" label in the fortress-mode top bar.
• Clicking an icon opens a searchable ListBox of the citizens in that stress category (e.g. "Citizens: Unhappy").
• Selecting a citizen dismisses the list and centers the map on them via dfhack.gui.revealInDwarfmodeMap.
• Empty categories show a brief message instead of an empty list.
• Icon positions are discovered each second by scanning the top bar for the Pop label, so the hotspot tracks window resizes and fort-name length changes. Only active on dwarfmode/Default.
• Includes a new in-engine test file (test/plugins/sort.lua) covering rect detection, click handling, list filtering, and map centering.

Test plan

• test -t plugins.sort in-game on a fortress save: 5/5 tests, 38/38 checks pass
• Clicked all seven icons in-game: populated categories open the filtered list; empty categories show the "no citizens" message
• Selecting a dwarf recenters the map viewport on their position (verified across z-levels)
• Clicks outside the icons pass through to the map; clicks on the modal list are consumed