docs: PRDs for local recording + mixer and studio quality tiers

PRD-local-recording.md: Dual-path architecture for podcast-quality
interviews — local lossless WAV recording alongside live call, with
sync markers for post-session alignment, resumable upload to a
self-hosted mixer service that produces normalized multi-track output.

PRD-studio-quality.md: Documents the Opus 32k/48k/64k studio tiers,
when to use them, cross-codec interop, and backward compatibility.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/PRD-studio-quality.md

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+# PRD: Studio Quality Tiers (Opus 32k/48k/64k)
+
+## Status: Implemented
+
+Studio quality tiers have been added to the wire protocol and all clients.
+
+## What Was Added
+
+### Wire Protocol (codec_id.rs)
+
+Three new `CodecId` variants using the 4-bit header space (values 6-8):
+
+| CodecId | Wire Value | Bitrate | Frame | Use Case |
+|---------|-----------|---------|-------|----------|
+| Opus32k | 6 | 32 kbps | 20ms | Studio low — noticeable improvement over 24k for voice |
+| Opus48k | 7 | 48 kbps | 20ms | Studio — excellent voice, captures nuance |
+| Opus64k | 8 | 64 kbps | 20ms | Studio high — near-transparent quality |
+
+### Quality Profiles
+
+| Profile | Codec | FEC | Bandwidth (with FEC) |
+|---------|-------|-----|---------------------|
+| STUDIO_32K | Opus 32k | 10% | ~35 kbps |
+| STUDIO_48K | Opus 48k | 10% | ~53 kbps |
+| STUDIO_64K | Opus 64k | 10% | ~70 kbps |
+
+FEC is set to 10% (vs 20% for GOOD) — studio assumes a good network.
+
+### Client Support
+
+| Client | Selection | Status |
+|--------|-----------|--------|
+| Desktop (Tauri) | Quality slider in Settings (8 levels) | Done |
+| CLI | `--profile studio-64k` / `studio-48k` / `studio-32k` | Done |
+| Android | Needs codec picker update in SettingsScreen.kt | TODO |
+| Web | Needs UI | TODO |
+
+### Cross-Codec Interop
+
+All decoder auto-switch paths (call.rs, desktop engine.rs) handle the new codec IDs. A studio-64k client can talk to a codec2-1200 client — the receiver auto-switches.
+
+## When to Use Studio Tiers
+
+- **Podcast recording sessions**: Use studio-64k for best quality (combined with local WAV recording for pristine output)
+- **Music collaboration**: Opus at 48-64k captures instrument harmonics much better than 24k
+- **Good network conditions**: Only useful when bandwidth isn't constrained; the extra bits are wasted on lossy networks
+
+## When NOT to Use
+
+- **Mobile data**: Stick with Auto/GOOD — studio tiers use 2-3x the bandwidth
+- **High packet loss**: Studio profiles use minimal FEC (10%); degraded networks need DEGRADED or CATASTROPHIC profiles with 50-100% FEC
+- **Large group calls**: Each participant's stream multiplies bandwidth; 64k * 10 participants = 640 kbps incoming
+
+## Backward Compatibility
+
+Old clients (before this change) will receive packets with CodecId 6/7/8 which they don't recognize. The `from_wire()` returns `None` for unknown values, causing the packet to be dropped. Old clients can still *send* to new clients fine (they use CodecId 0-5). This is acceptable for a pre-release protocol.