Walk and fly through the city

Adds walk and fly cameras alongside the helicopter view, switched with
1/2/3 or the city toolbar. Walking uses mouse-look with pointer lock
(or drag), WASD, Shift to run and circle-vs-footprint collision with
sub-stepping so long frames cannot tunnel through buildings; flying
follows the view pitch with Space/C for altitude and clears low roofs.
Double-clicking a street drops you there with an animated descent,
facing the longest open street. A crosshair names the building ahead
(E or click inspects it), the address bar shows the folder you are in,
and a minimap shows your heading and teleports on click.

web/index.html

@@ -81,6 +81,18 @@
           <span><b>Scroll</b> dolly</span>
           <span><b>F</b> reset</span>
         </div>
+        <div class="city-hud" id="cityHud" hidden>
+          <div class="city-views" role="group" aria-label="City camera">
+            <button type="button" data-city-view="heli" title="Helicopter (1)"><kbd>1</kbd> Helicopter</button>
+            <button type="button" data-city-view="walk" title="Walk (2)"><kbd>2</kbd> Walk</button>
+            <button type="button" data-city-view="fly" title="Fly (3)"><kbd>3</kbd> Fly</button>
+          </div>
+          <span class="city-help" id="cityHelp"></span>
+        </div>
+        <div class="crosshair" id="crosshair" hidden></div>
+        <div class="city-target" id="cityTarget" hidden></div>
+        <div class="city-address" id="cityAddress" hidden></div>
+        <canvas class="minimap" id="minimap" width="220" height="150" hidden title="Click to travel"></canvas>
         <nav class="breadcrumb" id="breadcrumb" aria-label="Selected file path">
           <button id="homeButton" aria-label="Fit entire codebase">←</button>
           <span id="breadcrumbText">No codebase loaded</span>