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/splitFlap.js

@@ -72,7 +72,7 @@ export const FLAP_STAGGER_MS = 26;
  */
 export const FLAP_MAX_TOTAL_MS = 620;
 /** Slack after the last character lands before the cells are stripped. */
-export const FLAP_SETTLE_SLACK_MS = 60;
+const FLAP_SETTLE_SLACK_MS = 60;
 /**
  * Fraction of a character's flip at which the outgoing glyph has rotated
  * away and the incoming one takes over as what the eye reads. MUST track the