feat(directions): keyless A→B directions with turn-by-turn steps and FLY (#564)

Supersedes #219 by @spcpza, 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: spcpza <[email protected]>

package.json

@@ -156,6 +156,7 @@
     "./layers/awareness": "./src/layers/awareness/index.js",
     "./layers/traffic": "./src/layers/traffic/index.js",
     "./layers/bikeshare": "./src/layers/bikeshare/index.js",
+    "./layers/directions": "./src/layers/directions/index.js",
     "./layers/cctv": "./src/layers/cctv/index.js",
     "./layers/alpr": "./src/layers/alpr/index.js",
     "./layers/radio": "./src/layers/radio/index.js",