Add wandering residents to the city

Little glowing green stick figures, in the spirit of Darwinia, stroll
the streets: each walks toward a random reachable point in plain sight
on the navigation grid, pauses, and about a third of the time heads
toward a nearby resident instead, so they gather in small crowds. They
are camera-facing sprites drawn procedurally in the fragment shader
with a walk cycle, flicker and halo, grown with altitude so crowds stay
visible from the helicopter, and fogged like everything else.

About one resident per 2,500 m² of city (40 to 600), spawned only on
the city's own streets. Because they keep the city animating, Settings
has a City residents switch (saved with the other preferences). The
simulation is seedable and tested to keep everyone on walkable ground.

web/index.html

@@ -161,6 +161,7 @@
       <div id="settingsContent"></div>
       <div class="preference-row"><label for="paletteSelect">Landscape palette<small>Color-code the layers of your codebase</small></label><select id="paletteSelect"><option value="0">Botanical</option><option value="1">Prismatic</option><option value="2">Autumn</option></select></div>
       <div class="preference-row"><label for="sourceToggle">Source text<small>Show code directly in the landscape</small></label><input type="checkbox" class="switch" id="sourceToggle" checked></div>
+      <div class="preference-row"><label for="residentsToggle">City residents<small>Little figures wandering the streets; keeps the city animating</small></label><input type="checkbox" class="switch" id="residentsToggle" checked></div>
       <div class="preference-row"><label for="fpsToggle">Performance monitor<small>Display frame rate on the canvas</small></label><input type="checkbox" class="switch" id="fpsToggle"></div>
       <div class="shortcut-guide"><span>Keyboard essentials</span><div><kbd>/</kbd> Search <kbd>F</kbd> Fit landscape <kbd>Esc</kbd> Close</div></div>
       <div class="settings-note">