Address Windows output endpoints by ID, not by name

[Turbootzz]

Closes #212. Both bugs are Windows-only, both silent, both from the output picker in #210.

The wrong endpoint

list_audio_outputs stored PKEY_Device_FriendlyName as the value the capture backend resolves. Identical hardware gives two endpoints the same friendly name, so picking the second of a pair resolved to the first and the loopback recorded an output nobody was listening to: clips with no system audio and nothing in the log to explain it. The picker now stores GetId() and shows the friendly name, which is what endpoint() already tried first.

The doubled mix

endpoint() fell back to the console default whenever a configured render endpoint matched nothing, deliberately: recording the current output beats recording silence when the user's pick disappears. That fallback also covered the internal communications-endpoint capture. If the comms endpoint went inactive between default_render_endpoints() reading it and the capture thread resolving its opaque ID (headset unplugged, comms default changed), the extra capture opened a second loopback on the console default — which the system capture is already recording, since comms is only spawned when the user picked no output. Both fed AudioMixer::add, so clips carried system audio at roughly double amplitude, clamped on drain into distortion, behind a single info log.

The fallback is right for one caller and wrong for the other, so the call now says which it means:

pub enum AudioDevice {
    Default,           // the platform default
    Preferred(String), // the user's pick; substituting the default beats silence
    Exact(String),     // this endpoint or nothing
}

capture_audio takes it in place of Option<&str> on all three backends. Windows falls back only for Preferred on the render flow; the comms capture passes Exact, so when its endpoint is gone the capture simply does not start — which main already tolerates, since it is spawned as an optional extra. Linux and macOS treat Preferred and Exact alike (PipeWire's session manager and SCK decide substitution themselves), documented at the enum.

Upgraded configs

A config written by an earlier build holds a friendly name. Capture still resolves it, through the same name match as before, so recording is unaffected. In the picker that value would have produced two identical rows (the real endpoint plus the "offline device" row that keeps an unlisted value visible), so the picker now matches a listed endpoint's label as well as its id; the stored value becomes an ID the next time anything is picked. Rewriting it on open is deliberately not done — the settings window only writes on Save, so that would show unsaved edits nobody made.

Verified

• cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, cargo build --workspace --all-targets, cargo test --workspace (23 test binaries, no failures) — plus three new tests covering the selector and which variants may be substituted.
• cargo check --target x86_64-pc-windows-msvc -p rewynd-capture -p rewynd-config for the two crates that hold the actual fixes. The app crate cannot cross-compile here (C build scripts want the MSVC toolchain), so its Windows block is on CI's windows job.

[Turbootzz]

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📥 Commits

Reviewing files that changed from the base of the PR and between 1e97388 and 5589586.

📒 Files selected for processing (8)

• crates/app/src/main.rs
• crates/capture/examples/audio_probe.rs
• crates/capture/src/lib.rs
• crates/capture/src/linux/audio.rs
• crates/capture/src/macos/audio.rs
• crates/capture/src/windows/wasapi_audio.rs
• crates/config/src/devices.rs
• crates/settings/src/main.rs

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