diag(audio,ios): instrument VPIO render callback to isolate playback breakage (rc.8+66)

User reports capture-side audio (mic -> remote) is clean but
local playback (remote -> speaker via VPIO render callback) is
'broken and poor' at +65. The pipeline appears correct on paper:
fill_buffer -> downmix (L+R)*0.5 -> gain -> clamp -> i16 -> VPIO.
No errors logged. To stop guessing, add structured logging
inside the render callback so the next test cycle yields data
about what's actually flowing through.

Diagnostic emitted every 100th callback (~2 s at iOS's typical
20-50 Hz callback rate):

  ios VPIO render callback diagnostic sample
    cb=<counter>
    num_frames=<N>           VPIO buffer size in mono samples.
                             Expected ~960 (20ms) or ~1104 (23ms).
                             Outliers point at format mismatch.
    peak_stereo_f32=<f32>    Peak |sample| of AudioHandler's
                             output BEFORE gain + downmix.
                             0.0 = handler is producing silence
                                   (jitter underrun, no audio).
                             ~1.0 = full-scale content reaching
                                    the callback as expected.
    peak_out_i16=<i16>       Peak |sample| of the downmixed mono
                             i16 we write to VPIO. Zero with
                             non-zero peak_stereo = downmix bug.
                             Near 32767 = clipping pressure.
    gain=<f32>               Current master output gain.

What we'll be able to diagnose from a 5-second talker session:

* peak_stereo_f32 = 0 throughout
    -> AudioHandler isn't producing samples. Inbound forwarder
       may not be feeding it, or jitter buffer is stuck in
       buffering_samples state. NOT a render-callback bug.

* peak_stereo_f32 oscillating, peak_out_i16 = 0
    -> Downmix or i16 cast is broken. Math bug in the loop.

* num_frames wildly different from ~960-1104
    -> StreamFormat got rejected and VPIO is delivering a
       different rate. Format-pinning fight with the session.

* peak_stereo_f32 normal AND peak_out_i16 normal AND user
  still says 'broken'
    -> The signal reaches the device cleanly but iOS's VPIO
       output processing (AEC residual subtraction, AGC
       compression, NS gate) is mangling it after our callback
       returns. That's a VPIO-config problem, not a render-
       callback problem; fix is to disable specific VPIO
       voice-processing properties on the unit before
       initialize().

Pure diagnostic commit. No behavioural change beyond a
warn-rate-limited info log line every ~2 seconds. Cost in the
audio thread is one branch + counter increment + (every 100th)
a tracing macro invocation.

Build counter 65 -> 66.

apps/chanora_flutter/pubspec.yaml

@@ -16,7 +16,7 @@ publish_to: 'none' # Remove this line if you wish to publish to pub.dev
 # https://developer.apple.com/library/archive/documentation/General/Reference/InfoPlistKeyReference/Articles/CoreFoundationKeys.html
 # In Windows, build-name is used as the major, minor, and patch parts
 # of the product and file versions while build-number is used as the build suffix.
-version: 1.0.0-rc.8+65
+version: 1.0.0-rc.8+66
 
 environment:
   sdk: ^3.11.5