fix(esp32/ledc): translate ledcWrite(pin,duty) → ledcWrite(channel,duty)

arduino-esp32 3.x ledcWrite takes a PIN and looks up the attached channel
internally. arduino-esp32 2.x (the toolchain version we pin) takes a
CHANNEL. The velxio_compat.h shim already aliased the 3.x-only
ledcAttach onto ledcSetup+ledcAttachPin so 3.x sketches would compile,
but ledcWrite still mapped 1:1 — so a call like

    #define R_PIN 16
    ledcAttach(R_PIN, 5000, 8);   // shim → channel 0 attached to pin 16
    ledcWrite(R_PIN, 128);        // ★ writes to "channel 16" (invalid)

silently wrote to LEDC channel 16, which doesn't exist (valid range
0-15). The hardware duty register never changed, qemu-lcgamboa never
emitted a `ledc_duty` event, and the RGB LED stayed dark even though
the firmware ran cleanly and the wires looked right. Verified end-to-end
with examples/esp32-pwm-led-rgb: gpio_change events fired at boot, no
ledc_duty events fired, ledRed/ledGreen/ledBlue all stayed at 0.

Fix: maintain a 40-entry pin→channel table populated by both ledcAttach
variants. Replace ledcWrite with a macro that calls a helper checking
the table first; if the value isn't a known pin we pass it through as a
channel, preserving 2.x channel-style call sites.

Macro/function name collision is sidestepped with the standard
parenthesizing trick — `(ledcWrite)(channel, duty)` doesn't expand the
function-like macro because the token isn't followed by `(`.

Verified live on velxio.dev/example/esp32-pwm-led-rgb after hot-copying
the new header into the velxio-app container: ledRed/ledGreen/ledBlue
now cycle through the full HSV wheel as expected (samples: (255,41,0),
(41,255,0), (0,41,255), (232,255,0), …).

Single-file sketch only — the table is `static` (internal linkage) and
ledcAttach + ledcWrite live in the header. Multi-file sketches that
attach in file A and write in file B would each see their own table.
Acceptable for now since arduino-esp32 sketches are nearly always
single-file; revisit when we bump the toolchain to 3.x and can drop the
shim entirely.

backend/app/services/esp-idf-template/main/velxio_compat.h

@@ -4,18 +4,41 @@
 // Compatibility shims for sketches written against arduino-esp32 3.x running
 // on the toolchain pinned to 2.0.17. Only kept until we bump the toolchain.
 //
-// ledcAttach / ledcAttachChannel were added in 3.x. Map them onto the 2.x
-// ledcSetup + ledcAttachPin pair so the sketch compiles unchanged.
+// arduino-esp32 3.x reshaped the LEDC API around pins:
+//   - ledcAttach(pin, freq, res)      // auto-allocates a channel
+//   - ledcAttachChannel(pin, freq, res, channel)
+//   - ledcWrite(pin, duty)            // takes pin, looks up channel
+//
+// arduino-esp32 2.x is channel-centric:
+//   - ledcSetup(channel, freq, res)
+//   - ledcAttachPin(pin, channel)
+//   - ledcWrite(channel, duty)
+//
+// We shim the 3.x calls onto the 2.x pair so unmodified 3.x sketches both
+// compile AND drive the LEDC peripheral correctly. A pin-to-channel table
+// keeps `ledcWrite(pin, ...)` working — without that translation
+// `ledcWrite(16, ...)` writes to LEDC channel 16 (invalid; channels are
+// 0-15) and the duty register never changes, so qemu-lcgamboa never emits
+// a duty event and the LED stays dark.
 
 #if defined(ARDUINO_ARCH_ESP32) && !defined(ledcAttach)
 #include "Arduino.h"
 
+// ESP32 / S2 / S3 / C3 all have <40 usable GPIOs. Sentinel 0xFF = unmapped.
+static uint8_t _velxio_pin_to_channel[40] = {
+    0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
+    0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
+    0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
+    0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
+};
+
 static inline bool ledcAttach(uint8_t pin, uint32_t freq, uint8_t resolution) {
     static uint8_t _velxio_next_channel = 0;
     if (_velxio_next_channel >= 16) return false;
     uint8_t ch = _velxio_next_channel++;
     ledcSetup(ch, freq, resolution);
     ledcAttachPin(pin, ch);
+    if (pin < 40) _velxio_pin_to_channel[pin] = ch;
     return true;
 }
 
@@ -23,8 +46,28 @@ static inline bool ledcAttachChannel(uint8_t pin, uint32_t freq, uint8_t resolut
     if (channel >= 16) return false;
     ledcSetup(channel, freq, resolution);
     ledcAttachPin(pin, channel);
+    if (pin < 40) _velxio_pin_to_channel[pin] = channel;
     return true;
 }
+
+// 3.x-style ledcWrite(pin, duty). Translate pin→channel via the table if
+// we know about this pin; otherwise fall through to 2.x channel semantics
+// so plain 2.x sketches keep working unchanged.
+static inline void _velxio_ledc_write(uint32_t pin_or_channel, uint32_t duty) {
+    if (pin_or_channel < 40) {
+        uint8_t ch = _velxio_pin_to_channel[pin_or_channel];
+        if (ch != 0xFF) {
+            // The extra parentheses around `ledcWrite` block macro expansion
+            // (function-like macros only expand when followed by `(`), so
+            // this calls the real 2.x ledcWrite from esp32-hal-ledc.h.
+            (ledcWrite)((uint8_t)ch, duty);
+            return;
+        }
+    }
+    (ledcWrite)((uint8_t)pin_or_channel, duty);
+}
+
+#define ledcWrite(pin_or_channel, duty) _velxio_ledc_write((pin_or_channel), (duty))
 #endif
 
 #endif // VELXIO_COMPAT_H