Recording: drop dry/wet Source choice, always write dry sidecar alongside wet

[Cheesewizard]

Goal

Remove the dry-vs-wet Source choice from recording. Every take records the wet game mix; a dry .dry.wav sidecar is written automatically alongside it whenever the dry input is ready. Simplifies both UIs and the flags plumbing, and means the dry take is always available (re-amping, NBN corpus) without deciding up front.

Why

• The Source selector is a per-take decision the user rarely wants to make, and picking dry means silently losing the wet take.
• Both signals are already observed live; only the file-writing side is mutually exclusive today.
• Removes UI + cross-process plumbing rather than adding it.

Current state (as of this ticket)

• Two orthogonal choices mirrored in the GUI Recording page (GUI/Audio/AudioRoutingPanel.cs) and the in-game overlay (DLL/Menu.cpp): Source (Game mix / Dry guitar) and Format (Audio WAV / Video MP4).
• Wet = output tap; dry = DrySignalRecording fed from Player 1's input hook (post-pitch-shift, pre-tone).
• On the ASIO path wet and dry share a single GameAudioRecorder slot (DLL/Audio/SharedOutput.cpp ~L377-401): op 2 opens it tagged "wet", op 14 opens it tagged "dry" and attaches the input tap, so they are mutually exclusive.
• Overlay flags word (DLL/Keybindings.cpp ~L75): bit 0x100 = override present, bit 1 = dry, bit 2 = video.

Proposed change

1. Recorder layer: add a second GameAudioRecorder slot so the output-fed (wet) and input-fed (dry) recorders can write concurrently. Wet is primary; dry is best-effort.
2. Readiness gating: only open the dry sidecar when DrySignalRecording::IsReady() (48k input packets within the last 1s). In cable mode / before the input hook is feeding, just write wet. Not lockstep with wet.
3. File naming: take.wav + take.dry.wav. TakeList should not clutter — group the sidecar under its parent or hide .dry.wav from the top level.
4. Video: unchanged — MP4 muxes the wet source; dry remains a sidecar wav.
5. UI removal: drop the Source segmented control (AudioRoutingPanel.cs), the Source radios (Menu.cpp), the RecordingSource persistence, overlay flags bit 1, and the wet/dry status strings.

Caveats / must-verify

• Confirm live that both recorders can write at once with no contention (two different sources: output tap vs input hook, so expected to be fine, but verify on a real ASIO session).
• Define behavior clearly when dry is not ready: wet-only take, no error surfaced as a failure.
• No functional loss: users who previously picked dry-only now also get the wet file (extra file, not a regression).

Notes

Request 1 (default recording to Video in both UIs) is already done in the working tree (uncommitted): Menu.cpp recFormat = 1, AudioRoutingPanel.cs new CaptureMode setting defaulting to Video. This ticket is the follow-up simplification.