fix: force currentMixerMode to MIXER_QUADX under USE_QUAD_MIXER_ONLY

CodeRabbit review on PR#1417 (github.com//pull/1417#issuecomment-5453591327):

mixerInit() set currentMixerMode from the persisted mixerConfig()
value unconditionally. config.c's validateAndFixConfig() -- which
would normally reset a stale mixer mode -- is itself skipped under
USE_QUAD_MIXER_ONLY. A device reflashed to a quad-only build with a
persisted non-quad mode (e.g. MIXER_TRI) would carry that value
into servoConfigureOutput()'s servoMixers[currentMixerMode] lookup
whenever a servo feature is active, loading the wrong servo rules
for the airframe actually being flown (motors hardcoded to QuadX,
servos configured per the stale mode). Reachable on a normal boot,
no exotic timing required.

Fix: mixerInit() forces currentMixerMode = MIXER_QUADX under the
flag, ignoring the persisted value -- single source of truth for
every downstream consumer (servos.c, mixerIsTricopter(), etc.).

Also (CodeRabbit P3, same review): the `smix` CLI help text still
advertised `load <mixer>` under USE_QUAD_MIXER_ONLY, where
cliServoMix() always rejects it with "Invalid name". Split into two
full CLI_COMMAND_DEF entries (embedding a preprocessor directive
inside a single macro call's argument list is non-portable and
produced its own new warning -- confirmed via build, reverted that
approach).

48/48 unit tests pass. Both configurations (with/without the flag)
build clean, zero warnings. Normal-build .elf size unchanged
(byte-identical to pre-fix).

Author: nerdCopter

src/main/flight/mixer.c

@@ -428,7 +428,12 @@ void initEscEndpoints(void) {
 }
 
 void mixerInit(mixerMode_e mixerMode) {
+#ifdef USE_QUAD_MIXER_ONLY
+    UNUSED(mixerMode);
+    currentMixerMode = MIXER_QUADX;
+#else
     currentMixerMode = mixerMode;
+#endif
     initEscEndpoints();
     if (mixerIsTricopter()) {
         mixerTricopterInit();

src/main/interface/cli.c

@@ -4602,10 +4602,16 @@ const clicmd_t cmdTable[] = {
     CLI_COMMAND_DEF("signature", "get / set the board type signature", "[signature]", cliSignature),
 #endif
 #ifdef USE_SERVOS
+#ifndef USE_QUAD_MIXER_ONLY
     CLI_COMMAND_DEF("smix", "servo mixer", "<rule> <servo> <source> <rate> <speed> <min> <max> <box>\r\n"
                     "\treset\r\n"
                     "\tload <mixer>\r\n"
                     "\treverse <servo> <source> r|n", cliServoMix),
+#else
+    CLI_COMMAND_DEF("smix", "servo mixer", "<rule> <servo> <source> <rate> <speed> <min> <max> <box>\r\n"
+                    "\treset\r\n"
+                    "\treverse <servo> <source> r|n", cliServoMix),
+#endif
 #endif
     CLI_COMMAND_DEF("status", "show status", NULL, cliStatus),
 #ifndef SKIP_TASK_STATISTICS