p4: T-Display P4 port — web VNC/remote, time-sync hardening, brightness + sound

LilyGo T-Display P4 (ESP32-P4 + factory ESP-AT C6) reaches POC: LoRa mesh, Wi-Fi
companion (TCP:5000), GPS, SD, battery gauge, chat persistence, and now:

- Web VNC/REMOTE/terminal via a first-byte router on the single ESP-AT listener
  (HTTP verbs -> WS server, '<' frames -> phone companion; both live at once).
  C6Server gains a _begun gate so never-listening instances stay inert.
- VNC freeze fixed: bounded one-CIPSEND-chunk drains under the WS client mutex
  (a whole-band blocking write starved the main loop), fast-drop of stalled
  mirror clients, SEND-OK wait 8s->4s.
- Wi-Fi robustness: CWJAP? miss-streak tolerance, CIPSERVER liveness verify +
  re-arm (a re-association silently killed the listener until reboot).
- Time sync: HTTP-Date fallback when SNTP is blocked; ClockFloorRTC gains a
  MAX_PLAUSIBLE_EPOCH guard (all boards) after a garbage-future hardware RTC
  read (2043) latched the ratchet.
- Brightness: RM69A10 DCS 0x51 driven live (CC slider + persisted pref).
- Sound: ES8311 + NS4150B notification chimes (P4Audio, esp-bsp-derived
  register sequence on Wire1), full HAS_UI_SOUND surface + CC Screen/Sound
  sliders; ccVolumeReleaseCb's hardcoded tanBeep -> uiSoundPreview.
- tdisplay_p4/: IDF project (build.sh, c6_at AT-over-SDIO driver, sdkconfigs,
  fetch-deps.sh vendoring, .gitignore for local payloads).

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

src/DataStore.cpp

@@ -5,7 +5,7 @@
 #include <SD.h>
 #include "helpers/esp32/WdtHeavyGuard.h"   // suspend core-0 idle WDT during the (SPIFFS-GC-prone) contact write
 #endif
-#if defined(HAS_TANMATSU)
+#if defined(HAS_TANMATSU) || defined(HAS_TDISPLAY_P4)
 #include <SD_MMC.h>
 #endif
 
@@ -835,8 +835,8 @@ bool DataStore::useSdStorage() {
 }
 #endif
 
-#if defined(HAS_TANMATSU)
-// Tanmatsu: same full-store adoption as useSdStorage(), but on the SD_MMC slot.
+#if defined(HAS_TANMATSU) || defined(HAS_TDISPLAY_P4)
+// P4 boards: same full-store adoption as useSdStorage(), but on the SD_MMC slot.
 // The internal FFat 'locfd' has a broken FAT metadata layer (f_stat/exists lie —
 // see the tile-cache notes), and the exists()-gated identity + prefs loads that
 // "worked" at -Og read different garbage at -Os and came up empty: fresh node