refactor(server): move the routing heap and the metre helpers out of the paddle domain

The binary heap and the flat-frame distance helpers are not about water;
the piste router needs the same two. They move to src/lib/geo, where any
domain may import them without reaching into paddle-routing. Nothing
changes in what they do.

server/src/lib/geo/geo.ts

@@ -1,4 +1,4 @@
-/** Metres per degree of latitude, the figure the paddle code reckons with throughout. */
+/** Metres per degree of latitude, the figure the routing code reckons with throughout. */
 const M_PER_DEG = 111_195;
 
 export function metresBetween(aLat: number, aLng: number, bLat: number, bLng: number): number {

server/src/nest/paddle-routing/build/build-state.ts

@@ -1,4 +1,4 @@
-import { lineLength, metresBetween, projectOnSegment } from '../engine/geo';
+import { lineLength, metresBetween, projectOnSegment } from '../../../lib/geo/geo';
 import { Access, EdgeKind, type EdgeKindCode, PortageSource } from '../water-graph';
 
 /** A node while the network is built: where it is, and the lake it opens into (-1 for none). */

server/src/nest/paddle-routing/build/lakes.ts

@@ -1,4 +1,4 @@
-import { LocalFrame } from '../engine/geo';
+import { LocalFrame } from '../../../lib/geo/geo';
 import type { LakeGrid } from '../water-graph';
 import { type BuildEdge, type BuildState, PointGrid } from './build-state';
 import type { CollectedArea, CollectedWater } from './collect';

server/src/nest/paddle-routing/build/portages.ts

@@ -1,4 +1,4 @@
-import { projectOnSegment } from '../engine/geo';
+import { projectOnSegment } from '../../../lib/geo/geo';
 import { Access, EdgeKind, PortageSource } from '../water-graph';
 import { type BarrierSite, type BuildEdge, type BuildState, PointGrid } from './build-state';
 import type { CollectedWater } from './collect';

server/src/nest/paddle-routing/engine/lake-field.ts

@@ -1,7 +1,7 @@
+import { LocalFrame } from '../../../lib/geo/geo';
+import { MinHeap } from '../../../lib/geo/min-heap';
 import { maskHas } from '../build/lakes';
 import type { LakeGrid } from '../water-graph';
-import { LocalFrame } from './geo';
-import { MinHeap } from './min-heap';
 import type { PaddleLakeStyle } from '@trek/shared';
 
 /**

server/src/nest/paddle-routing/engine/router.ts

@@ -1,8 +1,8 @@
+import { metresBetween, projectOnSegment } from '../../../lib/geo/geo';
+import { MinHeap } from '../../../lib/geo/min-heap';
 import { LakeIndex } from '../build/lakes';
 import { Access, EdgeKind, PortageSource, type WaterGraphData } from '../water-graph';
-import { metresBetween, projectOnSegment } from './geo';
 import { LakeField, type LakeProfile } from './lake-field';
-import { MinHeap } from './min-heap';
 import type {
   PaddleBoatType,
   PaddleLakeStyle,