Improve Atari Lynx Mikey timer and audio accuracy

[drhelius]

Improve Mikey timer and audio behavior in the Atari Lynx audio core.

Changes

• Store audio counter writes instead of ignoring them.
• Treat RESET DONE as a level, so it clears TIMER DONE while set.
• Trigger counter zero on the next input clock.
• Stop completed one-shot timers until TIMER DONE is cleared.
• Model clock selector 7 as a linked input instead of a fixed divider.
• Implement the complete Timer 1/3/5/7 and Audio 0/1/2/3 borrow ring.
• Handle CTLB software clocks and expose borrow, last-clock and timer-done status.
• Reload counters and advance the audio LFSR only on terminal borrow.
• Keep direct timers on a shared prescaler phase and service linked clocks in hardware order.
• Catch up sub-sample timer events before returning generated audio.
• Correct Lynx II stereo disable and attenuation routing.
• Fix timer, audio, attenuation, pan and stereo register readback.
• Keep LFSR state constrained to 12 bits and clamp the positive phase of volume $80.

These changes make timer state, linked clocks and audio output follow Lynx hardware behavior more closely.

This work is based on extensive hardware tests: https://github.com/drhelius/lynx-tests

There is a player implementation already using these changes here: https://www.drhelius.com/yawvgmp/

[drhelius]

Changed to draft to add more fixes, as I'm finding some more bugs, sorry about that.

[drhelius]

There were indeed many issues with the current implementation. Now it's ready.

[ValleyBell]

Are you sure about the return here?
In that case, it will pop a queue action and then discard it without any further processing.

I'd expect the code to instead compare against the front of the queue (without popping) and only pop when the item gets processed.

[drhelius]

My bad.

mikey_action_queue_pop is misnamed: it doesn't remove or modify the selected entry.

It only scans mTab and returns the earliest timestamp and its stage.

If the output buffer is full, the return leaves the queue unchanged, so the next mikey_update() selects the same action again. The entry is only replaced after mikey_pimpl_fireStage() processes it.

The check is before generating another sample so that an event reached by the final sample in the block can still be processed before returning.

I can rename it to mikey_action_queue_peek and make the queue pointer const.

[ValleyBell]

Please do that to reduce confusion. Thanks :)

[drhelius]

done!