fix(audio,ios): apply 8x output boost to compensate for VPIO raw playback (rc.8+69)

User-pasted log at +66 (https://pb.hit.moe/q8heratf.txt) shows
conclusive data over a 110-second continuous talker session:

  Average peak_stereo_f32: ~0.005-0.010
  Loud peak (one moment):  ~0.234
  peak_out_i16:           ~150-300 (out of 32767)

The signal arriving at our render callback from
AudioHandler::fill_buffer is consistently at -40 dB FS for
normal human speech. The Opus decode path in tsclientlib is
correct (Channels::Stereo decoder, no attenuation in fill_buffer,
queue.volume defaults to 1.0). The remote (official TS3 client)
is simply transmitting voice at the level desktop TS3 clients
typically do \u2014 well below speaker-ready amplitude.

On Linux/macOS/Windows our cpal+SDL output paths play that
signal through OS audio mixers that apply additional system-
volume amplification, reaching the user's ears at sensible
loudness. iOS's VPIO output is NOT amplified by the system
mixer \u2014 it goes nearly raw to the speaker, so the same -40
dB signal is barely audible. Musicbot (which encodes near full
scale at ~-6 dB) plays fine; human voice does not.

Fix: apply a fixed 8x (+18 dB) iOS output boost on top of the
existing user-controllable output_gain. A -40 dB signal becomes
-22 dB (normal speakerphone level). User's volume slider
continues to function in a useful 0-2x range on top.

  effective_gain = user_gain * IOS_OUTPUT_BOOST

Hard-clip at \u00b11.0 in the mono downmix prevents loud signals
(musicbot at peak 0.5 -> 4.0 -> clamped to 1.0) from
overflowing i16 wrap-around. Musicbot may distort on extreme
sustained content but voice remains intelligible at all
levels. Distortion ceiling matches the cpal-side FromF32 for
i16 conversion in engine.rs.

This is the same pattern Discord / Zoom / FaceTime iOS clients
apply: an internal output normalization on top of the user-
facing volume slider, calibrated so received voice is audible
at default settings.

Build counter 68 -> 69.

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+68
+version: 1.0.0-rc.8+69
 
 environment:
   sdk: ^3.11.5

crates/chanora_audio/src/ios_voice_unit.rs

@@ -598,18 +598,46 @@ impl IosVoiceUnit {
             // would otherwise clip). Multiplying by gain after
             // the downmix saves one multiplication per sample.
             //
+            // iOS-specific fixed output boost: AudioHandler delivers
+            // f32 stereo content matching what the remote client
+            // encoded. Most desktop TS3 clients ship audio at -30
+            // to -45 dB FS (peak ~0.005-0.03), well below speaker-
+            // ready levels. On Linux/macOS/Windows our cpal+SDL
+            // paths play that level through OS audio mixers that
+            // apply additional system-volume amplification, so it
+            // reaches the user's ears at sensible loudness. iOS's
+            // VPIO output is NOT amplified by the system mixer —
+            // it goes nearly raw to the speaker, so the same -40
+            // dB signal is barely audible.
+            //
+            // We compensate with a fixed 8x boost (= +18 dB) on
+            // top of the user-controllable output_gain. Brings a
+            // -40 dB signal up to -22 dB (normal speakerphone
+            // level) while keeping the user's UI gain slider
+            // functional in a useful range. Hard-clip at 1.0
+            // prevents the boost from clipping legitimate loud
+            // signals (musicbot at peak 0.5 -> 4.0 -> clamped to
+            // 1.0, audible distortion only on extremely loud
+            // sustained content).
+            //
+            // This is consistent with how Discord / Zoom / FaceTime
+            // iOS clients apply an internal output normalization
+            // on top of the user-facing volume slider.
+            const IOS_OUTPUT_BOOST: f32 = 8.0;
+            let effective_gain = gain * IOS_OUTPUT_BOOST;
+
             // Cast to i16 with saturate-on-overflow. Hard-clip is
             // acceptable here because the upstream signal is
             // already in [-1.0, 1.0] from the f32 stereo mix;
-            // gain values >1.0 are the only path to clipping and
-            // hard-clip at the engine boundary is what every other
-            // backend's i16 path does (see the cpal-side
-            // FromF32 for i16 impl in engine.rs).
+            // the IOS_OUTPUT_BOOST multiplier above is the
+            // primary path to clipping pressure and saturating
+            // at ±1.0 is the standard answer (matches the
+            // cpal-side FromF32 for i16 impl in engine.rs).
             let mut peak_out_i16: i16 = 0;
             for (i, dst) in out.iter_mut().enumerate() {
                 let l = scratch_stereo[i * 2];
                 let r = scratch_stereo[i * 2 + 1];
-                let mono_f32 = (l + r) * 0.5 * gain;
+                let mono_f32 = (l + r) * 0.5 * effective_gain;
                 let clamped = mono_f32.clamp(-1.0, 1.0);
                 let sample = (clamped * i16::MAX as f32) as i16;
                 *dst = sample;