feat(search): Nominatim as the last-resort geocoder (#573)

Supersedes #350 by @sendmebits, ported onto the current component layout with fixes from review.

Verified before merge: format, package boundaries, unit suite, build, and the browser tracking suite; a browser harness for the change where one applies; adversarial review applied.

Co-authored-by: sendmebits <[email protected]>

CHANGELOG.md

@@ -144,6 +144,28 @@ of current runtime behavior, see [`docs/CURRENT-STATE.md`](docs/CURRENT-STATE.md
   skip, so this affects every Overpass-backed layer: Mapped Installations,
   traffic roads and annotation geometry. Mirror rotation, cooldown and cache
   admission are unchanged (#420 — thanks @GladiatorrX9).
+- Place search has a last resort. With no Google Maps key, and when Photon does
+  not answer, a named-place search now falls back to OpenStreetMap's Nominatim
+  through `/api/geocode`, so search and voice fly-to still work on a keyless
+  globe. The route keeps to the service's usage policy: an identifying
+  User-Agent and Referer, at most one request per second, answers cached, one
+  upstream call shared between identical searches in flight, a bounded queue so
+  a burst is refused rather than held, and a queued search dropped once its
+  caller has given up (#350 — thanks @sendmebits).
+
+- Regional upstream reads now hold their deadline through the response body. The
+  abort timer was cleared as soon as the headers arrived, so an upstream that
+  answered and then stalled mid-body had no deadline at all. Redirect policy is
+  now stated per call rather than inherited, and the fixed Nominatim endpoints
+  refuse to be redirected.
+
+- The location search box answers two kinds of query without a network request
+  or an API key. A decimal-degree coordinate — `43.1731, -79.0384`, or either
+  order when N/S/E/W say which is which — flies straight there; a bundled city
+  or landmark name typed exactly (`paris`, `sf`, `Golden Gate Bridge`) flies to
+  the bundled place. Anything else, including anything malformed, goes to the
+  existing geocoders unchanged. Degrees/minutes/seconds and grid references are
+  not parsed and fall through the same way (#388 — thanks @KuraPiee).
 
 - Draped annotation geometry — area fills and outlines, routes and arrows —
   classifies onto terrain as well as 3D tiles. On a keyless boot, where Cesium's