fix(audio,ios): revert ring buffer back to direct fill_buffer call (rc.8+75)

The ring-buffer architecture (rc.8+73..+74) was making playback
strictly worse. Diagnostic data at +74 conclusively showed:

  * Producer task ran perfectly at 50 Hz (250 ticks per 5 s).
  * AudioHandler returned silence on 65-84% of fill_buffer calls
    even when window_peak_f32 reached 0.98 (full-scale audio).
  * Ring buffer never accumulated beyond 30 ms because consumer
    (VPIO render callback at 43.5 Hz, ~1440 samples per call)
    drained samples faster than the 50 Hz producer could push
    them, in net effect.

The producer drained AudioHandler at 50 Hz \u2014 slightly faster
than iOS VPIO actually consumes audio. Each fill_buffer call
asked for 20 ms but adjacent Opus packets hadn't arrived yet, so
fill_buffer returned mostly silence. Linux/SDL's same pattern
works because SDL calls fill_buffer at EXACTLY the device
callback rate (50 Hz = 20 ms per buffer); the rates match.

Fix: revert to direct fill_buffer call from the render callback
(the SDL pattern in tsclientlib's own reference example at
tsclientlib/examples/audio_utils/ts_to_audio.rs). The render
callback now:

  1. Resizes scratch_stereo Vec to 2 * num_frames f32 if needed
  2. Zeros the live slice (fill_buffer is additive, not clearing)
  3. Locks AudioHandler, calls fill_buffer(scratch_stereo)
  4. Downmixes L+R -> mono i16 with master gain into out[]
  5. Applies output_muted bypass
  6. Tracks peak_out + audio/silence ratios for diagnostic

The closure owns scratch_stereo across callbacks for stable
allocation. Same memory model as Linux/SDL.

Removed:
  * tokio::spawn producer task
  * rtrb dep + RingBuffer<i16> + Producer/Consumer split
  * tokio::sync::oneshot shutdown channel
  * producer_shutdown_tx field on IosVoiceUnit struct
  * RING_BUFFER_SAMPLES / PRODUCER_TICK_MS constants
  * Producer-side diagnostic counters

Diagnostic kept: cb / num_frames / frames_changes /
callbacks_with_audio / callbacks_with_silence / peak_out_i16 /
gain. Logged every 100 callbacks.

The choppy / clicks symptom is independent of the buffer
architecture \u2014 it's whatever AudioHandler is doing on iOS
that's different from Linux. Next investigation step is to
either (a) switch from VPIO to RemoteIO unit (lose Apple's
voice processing entirely), or (b) understand why AudioHandler
returns silence so often on iOS-arrival packet timing patterns.

Build counter 74 -> 75.

Cargo.lock

@@ -356,7 +356,6 @@ dependencies = [
  "ndk-context",
  "rand 0.8.6",
  "reqwest",
- "rtrb",
  "sdl2",
  "thiserror 2.0.18",
  "tokio",
@@ -2663,12 +2662,6 @@ dependencies = [
  "windows-sys 0.52.0",
 ]
 
-[[package]]
-name = "rtrb"
-version = "0.3.4"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4ade083ccbb4bf536df69d1f6432cc23deb7acccff86b183f3923a6fd56a1153"
-
 [[package]]
 name = "rusqlite"
 version = "0.32.1"