Add CYD 3.5C build: I2S on 22/21/35 with GT911 poll-only touch.

Export firmware/cyd_35r and document GPIO21 shared with GT911 INT.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/touch.cpp

@@ -6,7 +6,9 @@
  * 2.8": XPT2046 resistive SoftSPI bus (CS33).
  * 3.5C (3248S035C): GT911 capacitive on Wire1 SDA33/SCL32/INT21/RST25.
  * Feed coords are always native panel 320x480 for LVGL ROTATION_270.
- * Wire1 leaves Wire free for SGP30 (3.5: SDA22/SCL21; TFT_BL=27).
+ * Wire1 keeps GT911 off the INMP441 pins (WS22 / SCK21). After touch_init(),
+ * main claims GPIO21 as I2S BCK — touch stays poll-only (no INT IRQ).
+ * TFT_BL=27 on 3.5C.
  */
 
 static int16_t clampi(int32_t v, int32_t lo, int32_t hi) {
@@ -76,7 +78,7 @@ static void gt_hw_reset(bool addr_0x14) {
   delay(5);
   digitalWrite(CYD_GT911_INT, LOW);
   delay(50);
-  /* Backlight is GPIO27 — release INT(21) as input (may share SCL for SGP30). */
+  /* BL=27. Release INT(21) before main reuses it as INMP441 I2S SCK. */
   pinMode(CYD_GT911_INT, INPUT);
   delay(50);
 }