[djdefi]

Mixer::clip() overflow bug (audio pop/click)

clip() used 0x7FFFF (5 hex digits) as the positive clamp value. That
doesn't fit in the int16_t return type — it silently truncates to -1
on the implicit conversion. So whenever the mixed signal exceeds
+32767 (loud passages, or effects like Reverb/BitCrusher boosting
level), the mixer emits a raw -1 sample instead of clamping to full
volume, producing an audible pop/click.

Fixed to 0x7FFF with an explicit (int16_t) cast on the pass-through
path. Verified with an isolated reproduction of the old vs. new logic:
clip_old(40000) == -1, clip_new(40000) == 32767.

MixSystem::openChannel() — load a deck without touching the other

Added openChannel()/_openChannel() so a channel's source file can be
loaded or replaced while the mixer is already running, without
disturbing the other deck. Follows the existing request-queue pattern
(addSpeed/setEffect/etc.): routes through the audio thread via a new
MixRequest::OPEN when running, pausing only the target channel for
the swap.

This is needed by a companion JayD-Firmware change that lets
MixScreen start playing as soon as one deck is loaded, instead of
blocking until both decks are chosen.