PFD CDI softkey cycles nav source; demo reflects dataref writes

The CDI softkey was in the PFD root row but had no handler — it never cycled the
HSI/CDI source (GPS↔VLOC1↔VLOC2). Wire it to write cdiSrc
(HSI_source_select_pilot, now writable), cycling GPS→NAV1→NAV2.

Also fix a latent demo bug: the 'needs a live sim socket' guard sat above the
setDataref/command handlers, so in DEMO (no sim socket) every dataref write
(transponder code, baro, AP bugs, CDI) was silently dropped. Reflect writes
locally before that guard so cockpit controls respond without a sim.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/config.js

@@ -134,6 +134,7 @@ export const WRITABLE_DATAREFS = {
   apSpdBug: 'sim/cockpit2/autopilot/airspeed_dial_kts_mach',
   xpdrMode: 'sim/cockpit2/radios/actuators/transponder_mode', // 0 off,1 stby,2 on,3 alt
   xpdrCode: 'sim/cockpit2/radios/actuators/transponder_code', // 4-digit squawk
+  cdiSrc:   'sim/cockpit2/radios/actuators/HSI_source_select_pilot', // 0 NAV1 · 1 NAV2 · 2 GPS (CDI softkey cycles it)
 };
 
 // Commands the frontend may TRIGGER (autopilot mode buttons etc.).