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/spawn.rs

@@ -6,10 +6,6 @@
 //! [`SpawnEvent`]s a reader thread hands to the app's navigation closure: spinner until
 //! `{"ready":true}`, banner from the `{"error"|"ended": …}` line, `trust_rejected`
 //! routed to the re-pair PIN ceremony, `stats:` lines to the session status page.
-//!
-//! The legacy in-process D3D11VA presenter remains reachable via the
-//! `PUNKTFUNK_BUILTIN_STREAM=1` env override (`app::use_builtin_stream`) — the
-//! developer A/B baseline until its deletion.
 
 use std::io::BufRead as _;
 use std::process::{Child, Command, Stdio};