chore: drop exports nothing imports

Knip, configured with the application, package exports, scripts, QA
harnesses, workers, tools and tests as entry points, reports exports
that no module imports. Each was checked by hand against string-based
and namespace imports.

- Symbols that are still used inside their own module lose only the
  `export` keyword (176 constants and helpers).
- Unused re-exports are removed from barrel modules (flight, military,
  vessel, CCTV, directions, transit history, local ADS-B and street-level
  policy modules).
- Declarations that nothing uses at all are deleted, among them
  AIS_WATCHDOG_STATUSES, applyFilter, isRadioCountryCode, compactText,
  DENSITY_STOPS, the unused CCTV endpoint constants, and the SDR band
  constants.
- Default exports that nothing imports are removed. The ones in
  src/data/earthquakes.js, src/data/telegeographySubmarineCables.js,
  src/data/wind.js and src/data/bhoteKoshiLocator.js each constructed a
  layer instance at import time that nothing used; those instances are
  no longer created.

Every export of a package.json entry stays, as does TOMTOM_CREDIT,
which the traffic layer reads from the credits namespace it is given.

src/data/gtfsRealtime.js

@@ -35,14 +35,14 @@
 import { PbfReader } from 'pbf';
 
 /** VehiclePosition.VehicleStopStatus enum → label. */
-export const VEHICLE_STOP_STATUS = Object.freeze({
+const VEHICLE_STOP_STATUS = Object.freeze({
   0: 'INCOMING_AT',
   1: 'STOPPED_AT',
   2: 'IN_TRANSIT_TO',
 });
 
 /** VehiclePosition.OccupancyStatus enum → label. */
-export const OCCUPANCY_STATUS = Object.freeze({
+const OCCUPANCY_STATUS = Object.freeze({
   0: 'EMPTY',
   1: 'MANY_SEATS_AVAILABLE',
   2: 'FEW_SEATS_AVAILABLE',
@@ -64,7 +64,6 @@ export const GTFS_MAX_ENTITIES = 50_000;
 export const GTFS_MAX_STRING_CHARS = 256;
 /** FeedHeader.Incrementality: only FULL_DATASET carries a complete snapshot. */
 export const GTFS_INCREMENTALITY_FULL_DATASET = 0;
-export const GTFS_INCREMENTALITY_DIFFERENTIAL = 1;
 
 /**
  * Read a length-delimited string, refusing one that is implausibly long.