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

[Alistair-Afton]

Windows test-leg failures are the known DF startup crash (DF exited with 3221226356 at launch, before plugins load) — same signature appears in passing runs' logs, e.g. the fix-workshop-search-names run. Rebased onto develop (which now includes the #5943 retry fixes) to retrigger.

[Alistair-Afton]

Correction to the above: the Windows failures were not the startup crash — the tests ran (310/314) and errored on icon_rects being nil. On Windows CI the window is hidden, so the vanilla top bar never renders and the Pop text the scan anchors on is absent. Fixed in 7d9af7f: the wrapper waits ~20 frames for the top bar, marks scanning unavailable when absent, and the click/list/reveal tests inject synthetic rects so the logic is still exercised. CI is now green on all legs.