Merge branch 'gemini/doc-adaptive-jitter': document adaptive playout-delay controller

docs/ARCHITECTURE.md

@@ -60,7 +60,7 @@ All source code resides inside `src/`:
 * **`src/core/`**
   * **[src/core/mod.rs](file:///home/mollusk/git/butter/peerspeak/src/core/mod.rs)**: Implements the central room coordination, mixer logic, voice activity noise-gating, and the session shutdown sequencer.
   * **[src/core/messages.rs](file:///home/mollusk/git/butter/peerspeak/src/core/messages.rs)**: Formulates the inter-thread messaging schemas (`CoreCommand` and `UiEvent`).
-  * **[src/core/jitter.rs](file:///home/mollusk/git/butter/peerspeak/src/core/jitter.rs)**: Provides a per-peer jitter buffer that reorders out-of-order frames, drops duplicate arrivals, and handles packet drops.
+  * **[src/core/jitter.rs](file:///home/mollusk/git/butter/peerspeak/src/core/jitter.rs)**: Provides a per-peer jitter buffer that reorders out-of-order frames, drops duplicate arrivals, and handles packet drops. It also holds an *adaptive* playout delay that grows on jitter/loss and shrinks on a sustained clean run (see Section 4).
 * **`src/network/`**
   * **[src/network/mod.rs](file:///home/mollusk/git/butter/peerspeak/src/network/mod.rs)**: Formulates abstractions for low-latency P2P transports.
   * **[src/network/iroh_impl.rs](file:///home/mollusk/git/butter/peerspeak/src/network/iroh_impl.rs)**: Integrates Iroh's endpoint system. Orchestrates connection supervisors, dial routines, and handshake logic.
@@ -114,7 +114,60 @@ Audio processing runs on a 20ms clock (960 samples at 48kHz mono S16LE):
 
 ---
 
-## 4. Network and P2P Transport
+## 4. Adaptive Playout Delay Controller
+
+To absorb network jitter (packet arrival variation) without incurring unnecessary latency, PeerSpeak replaces a fixed playout buffer depth with an adaptive feedback controller. The controller operates purely on buffer observations without requiring wall-clock synchronization or NTP.
+
+### Controller State & Parameters
+
+The controller tracks three key metrics per peer inside the [JitterBuffer](file:///home/mollusk/git/butter/peerspeak/src/core/jitter.rs#L62) struct in [src/core/jitter.rs](file:///home/mollusk/git/butter/peerspeak/src/core/jitter.rs):
+* **Target Delay (`target_delay`)**: The number of frames that must accumulate in the buffer before playout is allowed to start (or restart after an underrun). Clamped to `[MIN_DELAY_FRAMES, MAX_DELAY_FRAMES]` (i.e., `[2, 12]` frames or ~40–240ms). Defaults to `DEFAULT_DELAY_FRAMES` (`3` frames, ~60ms).
+* **Clean Run (`clean_run`)**: The number of consecutive real, in-order frames played without disruption. Used to drive the slow shrink phase.
+* **Buffering Ticks (`buffering_ticks`)**: The number of 20ms mixer ticks spent in the buffering state since the first packet arrived.
+
+---
+
+### Feedback Loop and Delay Adjustments
+
+The controller employs a fast-grow / slow-shrink strategy: it reacts immediately to network disruptions (a one-frame increase per disruption event) but reclaims latency cautiously (a one-frame decrease, and only after a long clean run). Both adjustments are additive — the shrink's slowness comes from the `CLEAN_RUN_TO_SHRINK` gate, not from multiplicative scaling.
+
+```
+                        ┌────────────────────────┐
+                        │      Packet Event      │
+                        └───────────┬────────────┘
+                                    │
+           ┌────────────────────────┼────────────────────────┐
+           ▼ (Late/PLC Gap)         ▼ (Clean Playout)        ▼ (Silence/Underrun)
+┌──────────────────────┐ ┌──────────────────────┐ ┌──────────────────────┐
+│  Grow Target (+1)    │ │ Increment clean_run  │ │  Reset clean_run     │
+│  Reset clean_run     │ │ If clean_run >= 250: │ │  Go Idle (re-buffer) │
+└──────────────────────┘ │  Shrink Target (-1)  │ │  Target Unchanged    │
+                         │  Reset clean_run     │ └──────────────────────┘
+                         └──────────────────────┘
+```
+
+1. **Grow (Fast Increase)**:
+   * **Late Arrival**: If a packet arrives with a sequence number older than the next sequence to be played (`seq < next_seq`) in [insert](file:///home/mollusk/git/butter/peerspeak/src/core/jitter.rs#L121), it is discarded, and target delay grows by 1 frame (up to `12`) via [note_disruption](file:///home/mollusk/git/butter/peerspeak/src/core/jitter.rs#L104).
+   * **PLC Gap**: If the mixer polls a frame in [pop_frame](file:///home/mollusk/git/butter/peerspeak/src/core/jitter.rs#L144), the next sequence is missing, but later packets are already buffered, the decoder performs Packet Loss Concealment (PLC), and target delay grows by 1 frame (up to `12`) via [note_disruption](file:///home/mollusk/git/butter/peerspeak/src/core/jitter.rs#L104).
+   * *Rationale*: The jitter exceeded the current cushion. Growing the target delay ensures a deeper cushion next time the buffer primes.
+2. **Shrink (Slow Decrease)**:
+   * **Clean Playout**: Each time a real, in-order frame is successfully popped and played in [pop_frame](file:///home/mollusk/git/butter/peerspeak/src/core/jitter.rs#L144), the controller increments `clean_run` via [note_clean](file:///home/mollusk/git/butter/peerspeak/src/core/jitter.rs#L111).
+   * When `clean_run` reaches `CLEAN_RUN_TO_SHRINK` (`250` frames, or ~5 seconds of continuous playback), the target delay is reduced by 1 frame (down to `2`), and `clean_run` is reset.
+   * *Rationale*: The link has been stable for a sustained period, indicating we can safely reclaim latency.
+3. **Silence Neutrality**:
+   * If a talker pauses speaking, the buffer runs dry and goes idle. This is a benign underrun rather than a network anomaly.
+   * The controller transitions the playout state to buffering (`next_seq = None`) and resets `clean_run`, but **leaves `target_delay` untouched**. This avoids inflating playout latency during natural pauses in conversation.
+4. **Buffer Overflow Resync**:
+   * If the number of buffered packets exceeds `MAX_BUFFERED_FRAMES` (`32` frames, ~640ms), the buffer has fallen too far behind.
+   * The oldest frame is discarded, the playout head (`next_seq`) is resynced to the oldest remaining packet sequence, and `clean_run` is reset. No change is made to `target_delay`.
+5. **Prime-Timeout Safety Net**:
+   * When transitioning from idle to buffering, the mixer polls every 20ms and increments `buffering_ticks` if packets are present but their count is below `target_delay`.
+   * If `buffering_ticks` reaches `PRIME_TIMEOUT_TICKS` (`25` ticks, ~500ms), playout is primed and started immediately, even if the buffer length has not met `target_delay`.
+   * *Rationale*: This prevents short utterances (e.g., "yes", "okay") from being held indefinitely in a deep target cushion, capping the worst-case startup latency.
+
+---
+
+## 5. Network and P2P Transport
 
 P2P voice chats utilize a dual-plane architecture:
 1. **Control Plane (iroh-gossip)**: Distributes room roster join/leaves and audio state updates (mute/deafen) using Gossip.
@@ -130,7 +183,7 @@ To ensure reconnection reliability, the dialer caches each peer's complete `Endp
 
 ---
 
-## 5. Reconnect and Eviction Model
+## 6. Reconnect and Eviction Model
 
 Connection drops are managed via a dual-path armed grace timer to maintain a stable room state:
 
@@ -153,7 +206,7 @@ Connection drops are managed via a dual-path armed grace timer to maintain a sta
 
 ---
 
-## 6. Core loop & UI Flow
+## 7. Core loop & UI Flow
 
 The core async orchestrator in `src/core/mod.rs` schedules five main concurrent tasks inside an `ActiveSession`:
 * **`event_task`**: Processes gossip events, peer updates, and room roster modifications.
@@ -166,7 +219,7 @@ Updates are forwarded back to the Iced GUI via `UiEvent` channels, keeping room
 
 ---
 
-## 7. Config and Privacy
+## 8. Config and Privacy
 
 * **Default Privacy**: PeerSpeak sets the `NetworkMode` config to `RelayNoDiscovery` by default. This forces P2P connectivity through Iroh's relay nodes for NAT traversal without publishing the peer's endpoint metadata to the public n0 DNS directory.
 * **Persisted UI State**: Configuration values (devices, noise gates, custom chime file paths) are saved once when the settings page is exited via `AppMessage::NavigateBack`.