APU: various channel bugs (sequencer reset, length counter gating, timer stepping, noise LFSR) #609
Description
Summary
Several APU channel bugs:
1. Square sequencer not reset on $4003/$4007 write
Per nesdev APU Pulse, writing the 4th register ($4003 or $4007) resets the duty cycle sequencer to step 0. The code (channel-square.js:158-168) only sets envReset = true but never resets squareCounter. This affects note attack timing.
2. Triangle/Noise length counters loaded when channel disabled
channel-triangle.js:72 and channel-noise.js:85 load the length counter without checking isEnabled. Per the wiki, the length counter should only be loaded when the channel's enable flag ($4015) is set.
3. Square timer uses if instead of while (papu/index.js:256, 266)
The triangle channel correctly uses while for its timer, but the square channels use if. At high frequencies with long CPU instructions, multiple sequencer steps can be missed within a single timer period.
4. Noise LFSR shifts left instead of right (papu/index.js:286)
Real hardware shifts the 15-bit LFSR right with feedback from bit 0. The code shifts left and tests bit 15. While both produce pseudo-random sequences, the exact output differs.