feat(geo): add src/geo, Cesium's vector maths without Cesium

An engine-neutral port of the CesiumJS 1.138 core maths gev uses: angle
maths, the WGS84 ellipsoid and geodetic coordinates, 2D/3D vectors, 3x3/4x4
matrices, quaternions and local east-north-up / heading-pitch-roll frames.
Formulas and operation order match Cesium, so outputs are bit-identical
(golden tests pin the centre, pole, -0 and quaternion branches).

- Values are plain-data instances of module-private constructors (own
  enumerable fields only), so geo's hot functions keep their own V8 hidden
  classes; matrices are plain column-major arrays Cesium accepts.
- Cesium's debug argument checks are ported with the same conditions, order
  and DeveloperError messages, gated on DEBUG (src/geo/debug.js): on in Node
  and the dev server, folded away by vite build, as Cesium's Build strips
  them. A release-parity test compares DEBUG=false geo with Build/Cesium.
- The import-direction check forbids cesium and @cesium/* in src/geo;
  CODE-BOUNDARIES.md and THIRD_PARTY_NOTICES.md document the port.

THIRD_PARTY_NOTICES.md

@@ -2,8 +2,8 @@
 
 The project depends on third-party npm packages whose own licenses apply. This
 file records the packages added for the browser-local SDR feature and the
-Street Level viewer; the complete resolved dependency inventory remains in
-`package-lock.json`.
+Street Level viewer, and the CesiumJS code ported into `src/geo/`; the complete
+resolved dependency inventory remains in `package-lock.json`.
 
 ## Web RTL-SDR
 
@@ -45,3 +45,23 @@ Street Level viewer; the complete resolved dependency inventory remains in
 - Source: <https://github.com/mrdoob/three.js>
 - License: MIT
 - License text: <https://github.com/mrdoob/three.js/blob/dev/LICENSE>
+
+## CesiumJS (portions ported to src/geo)
+
+- Package: `cesium` 1.138.0 (`@cesium/engine` 22.3.0); the files in `src/geo/`
+  are adapted from `@cesium/engine/Source/Core` (`Math.js`, `Ellipsoid.js`,
+  `scaleToGeodeticSurface.js`, `Cartographic.js`, `Cartesian2.js`,
+  `Cartesian3.js`, `Matrix3.js`, `Matrix4.js`, `Quaternion.js`,
+  `Transforms.js`, `Check.js`, `DeveloperError.js`) and say so in their first
+  line. The changes: the classes' static functions became module functions,
+  vectors, geodetic positions and quaternions became plain-data instances of
+  module-private constructors (own enumerable fields only, no methods),
+  matrices plain arrays, and the ellipsoid a frozen object; the debug
+  argument checks were ported with Cesium's conditions and messages and, like
+  Cesium's, are active in development and tests and stripped from production
+  builds; only the subset gev uses was kept.
+- Author: Copyright 2011-2024 CesiumJS Contributors
+- Source: <https://github.com/CesiumGS/cesium>
+- License: Apache License 2.0
+- License text: `node_modules/@cesium/engine/LICENSE.md` (shipped with the
+  dependency) and <https://www.apache.org/licenses/LICENSE-2.0>