feat: record PipeWire audio as Ogg on Linux

[pranc1ngpegasus]

What changed

• add a Linux PipeWire capture backend for the default microphone and optional default-sink monitor
• record the two sources as separate mic.ogg and system.ogg Ogg/Opus streams
• integrate Linux record-only behavior into the existing Session facade used by the desktop runner
• preserve a shared monotonic timeline with bounded silence compensation for late starts, pauses, and queue overflow
• use bounded, non-blocking capture queues and gracefully drain/finalize both streams on stop
• validate negotiated PCM and buffer metadata, propagate required-microphone and writer failures, and bound malformed overflow handling
• create recording files exclusively with owner-only permissions
• add PipeWire/libclang Nix and Debian/Ubuntu build dependencies plus Linux usage documentation
• add hardware-free routing, alignment, Ogg decode, lifecycle, policy, overflow, and security tests; include an opt-in virtual PipeWire smoke test

Why

Linux previously returned UnsupportedPlatform. This reaches the first usable Linux milestone: Wisp can capture local microphone audio and, where PipeWire exposes it, system audio, and persist valid Ogg files. Linux transcription remains explicitly out of scope and the session runs record-only.

Impact

On Linux, the existing desktop session path can now start and stop PipeWire recording. mic.ogg is required; system.ogg is always a valid aligned stream and contains silence when no sink monitor is available. macOS and Windows behavior is preserved.

Validation

• cargo test -p wisp-audiokit — 127 unit tests passed
• doc tests — 2 passed
• cargo clippy -p wisp-audiokit --all-targets -- -D warnings
• cargo fmt --all
• git diff --check
• nixfmt --check flake.nix nix

The local host is macOS, so Linux-only compilation and the ignored virtual-PipeWire smoke test could not run locally. The Ubuntu nix flake check PR job is the Linux-native compile/clippy/test gate for this draft.