Rename Demolish to Deconstruct; add build button grid icons

Rename the Demolish* terminology to Deconstruct* across the simulation,
events, UI, tests, and docs (DemolishCommand and the DemolishMode* events
become their Deconstruct* equivalents), aligning with the deconstruction
queue at HEAD.

Add per-building SVG icons for the build button grid (REQ-UI-BUILD-GRID):
one color-chip icon per placeable building under data/icons/buildings,
loaded from disk at runtime and rendered with QtSvg onto each build button.
An unaffordable building's button shows a grey-recolored variant via a
QIcon Disabled-mode pixmap (REQ-UI-BUILD-DISABLED). Adds the Qt5::Svg
dependency and deploys Qt5Svg.dll.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y7N59FsLA5e2kuVdqe4Uhc

src/test/BeltSystemTest.cpp

@@ -974,7 +974,7 @@ TEST_CASE("BeltSystem: unpaired entry blocks items at front", "[belt]")
     REQUIRE(bs.peekItem(Port{QPoint(3, 0), Rotation::East}).has_value());
 }
 
-TEST_CASE("BeltSystem: demolish entry discards transit items", "[belt]")
+TEST_CASE("BeltSystem: deconstruct entry discards transit items", "[belt]")
 {
     BeltSystem bs(kFastBeltSpeed);