fix(apu): correct frame counter timing, add LP filter, mute aliased triangle

- Fix frame counter running at 2× speed: clock_frame_counter now skips
  odd CPU cycles (APU cycle = CPU/2), so envelope, sweep, and length
  counters tick at the correct rate. Fixes sweep-driven whistle in Megaman II.

- Switch audio sampling to per-CPU-cycle granularity in
  run_until_frame_complete_with_audio to eliminate square-wave harmonic
  aliasing caused by sampling only once per instruction.

- Add IIR one-pole low-pass filter (~14 kHz) to AudioMixer to smooth
  abrupt level transitions (crackling) introduced by correct envelope timing.

- Mute triangle channel when timer_period < 2 (≥27 kHz), which aliases
  into the audible range at 48 kHz. Real NES RC circuit removes these
  ultrasonics; emulator must suppress them explicitly.

- Update all APU bus tests to use correct (doubled) CPU cycle counts.

src/native_core/apu/api.rs

@@ -331,9 +331,13 @@ impl Apu {
         };
 
         let triangle = {
+            // Timer period < 2 produces ultrasonic output (~28-56 kHz) that aliases
+            // to audible frequencies when sampled at 48 kHz. Real hardware filters
+            // this via the RC output stage; mute here to match that behaviour.
             let active = (self.channel_enable_mask & 0x04) != 0
                 && self.length_counters[2] > 0
-                && self.triangle_linear_counter > 0;
+                && self.triangle_linear_counter > 0
+                && self.triangle_timer_period() >= 2;
             if active {
                 TRIANGLE_SEQUENCE[self.triangle_step as usize & 0x1F]
             } else {