refactor(windows): remove the legacy in-process builtin stream path

The real Windows client is the spawned punktfunk-session Vulkan binary
(pf-client-core); the in-process builtin GUI stream — reachable only via
PUNKTFUNK_BUILTIN_STREAM=1 — was dead weight kept alive by nothing and a
recurring source of wasted effort. Remove it: delete present/render/input/
audio.rs and the builtin remainder of session/video.rs, rip all the builtin
wiring (app/mod, connect, stream), and make connect always spawn.

Preserve the two shipped keepers that happened to live in those files by
relocating them to a new probe.rs: run_speed_probe (the per-host network speed
test used by the Settings speed page and --headless --speed-test) and
decodable_codecs (the codec-capability advert on the probe connect). Trim gpu.rs
to just the Settings adapter picker (adapter_names + helpers). --headless now
supports only --speed-test — the in-process decode/frame-counter went with the
pump.

Drops the now-orphaned deps opus, wasapi, crossbeam-channel, anyhow; keeps
ffmpeg-next (probe::decodable_codecs still needs it). Net 4432 deletions.
Statically verified (module wiring, imports, orphaned symbols/deps all clean);
the type-level compile runs on the windows-amd64 CI runner, which has the
toolchain this non-Windows host lacks.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

clients/windows/src/app/help.rs

@@ -1,8 +1,7 @@
 //! The Shortcuts screen: a short note on the in-stream capture model plus a reference of the
 //! keyboard shortcuts — reached from the Shortcuts button on the host list. The Windows
 //! counterpart of the GTK client's Keyboard Shortcuts window; the bindings themselves live in
-//! the session window (and [`crate::input`] for the legacy builtin path), so both clients
-//! document the same set.
+//! the session window, so both clients document the same set.
 
 use super::style::*;
 use super::Screen;
@@ -10,8 +9,7 @@ use windows_reactor::*;
 
 /// The in-stream keyboard shortcuts, in the GTK Shortcuts window's order: the chord, then what it
 /// does. Read-only — the keyboard bindings live in the session window (`pf-presenter`'s run
-/// loop; the legacy builtin path's in [`crate::input`]), the controller chord in its gamepad
-/// service.
+/// loop), the controller chord in its gamepad service.
 const STREAM_SHORTCUTS: &[(&str, &str)] = &[
     ("F11 / Alt+Enter", "Toggle fullscreen"),
     (