chore: drop unused exports from modules QA harnesses load

QA harnesses import some modules in the page by path, for example
import('/src/data/radio.js'). Those calls name exactly which bindings
they use, and every other export of these modules has no importer:

- src/data/radio.js: 20 aliases of radio layer methods and two
  re-exported altitude constants. The radio layer keeps the methods.
- src/layers/transit/movement.js: tuning constants and course helpers
  used only inside the module lose `export`; FIX_HISTORY_MAX,
  DISPLAY_LAG_MIN_MS, DISPLAY_LAG_RESET_MS, FIX_NOISE_M, STOPPED_FIXES
  and courseBetween, which nothing reads, are deleted.
- src/overlays/worldOverlay.js, src/splitFlap.js,
  src/data/contactPlayback.js and src/voice/cloudVoiceAuth.js: helpers
  and constants used only locally lose `export`.

src/data/contactPlayback.js

@@ -181,7 +181,7 @@ export function trimTrack(track, nowMs, keep = track.capacity) {
   for (const id of track.epochs.keys())
     if (!used.has(id)) track.epochs.delete(id);
 }
-export function validFix(fix) {
+function validFix(fix) {
   return (
     Number.isFinite(fix?.t) &&
     fix.t >= 0 &&