t096: shared PWM brightness for the heartbeat/TX LED (unpersisted)

Single LED (P0.28) doubles as heartbeat, message flash, shutdown flash
and LoRa TX indicator, all sharing the same physical pin. Adds one PWM
channel on that pin (heartbeat-pwm-led/lora-tx-pwm-led aliases) and a
single 0-100 brightness knob (get/set led) covering every one of those
events at once, deliberately RAM-only: resets to a 10% default on every
reboot regardless of CLI changes made in a previous session. The
existing persisted on/off gate (set leds on|off) is untouched.

Boards without heartbeat-pwm-led/lora-tx-pwm-led aliases keep their
current plain-GPIO behavior unchanged.

zephcore/helpers/led_gate.h

@@ -34,6 +34,19 @@ bool zephcore_leds_disabled(void);
  * progress. Safe to call from any role, with or without a UI. */
 void zephcore_leds_set_disabled(bool disabled);
 
+/* Shared PWM brightness (0-100) for boards whose heartbeat/TX LED is
+ * PWM-capable (DT_ALIAS(heartbeat_pwm_led) / DT_ALIAS(lora_tx_pwm_led)).
+ * Deliberately RAM-only, not persisted: resets to
+ * ZEPHCORE_LED_DEFAULT_BRIGHTNESS_PCT on every reboot regardless of any
+ * "set led <pct>" done during a previous session. Independent of the on/off
+ * gate above: brightness only matters while the gate is open. No-op on
+ * boards without a PWM-capable LED (the getter still returns the compiled
+ * default, simply unused). */
+#define ZEPHCORE_LED_DEFAULT_BRIGHTNESS_PCT 10
+
+uint8_t zephcore_led_brightness_pct(void);
+void zephcore_led_set_brightness_pct(uint8_t pct);
+
 /* Called by zephcore_leds_set_disabled() after the flag changes. Weak no-op in
  * led_gate.c; helpers/ui/ui_common.c overrides it to stop/restart the heartbeat
  * cycle and refresh the UI's LED page. Not meant to be called directly. */