feat(windows-host): finish HostConfig migration — resolve operator/dispatch knobs once (Goal-1 stage 2)

Migrate 31 genuinely-constant operator/dispatch env::var sites onto HostConfig, so the
capture/topology/encoder decision reads ONE owner instead of being recomputed at each call
site (the latent bug where capture and encode could disagree on the resolved backend, plan §2.4):
idd_push x7, no_wgc, capture_backend, render_adapter, encoder_pref (Linux open_video +
linux_zero_copy_is_vaapi), the Windows vdisplay-backend select, plus the plan-named
secure_dda/idd_depth/zerocopy/ten_bit and the multi-site perf x4 / compositor x5 /
video_source x3 / gamepad. Each HostConfig field's parser is byte-identical to the read it
replaced, so old==new by construction (the plan's "a flipped bool is a silent regression" guard).

Scope correction — the plan's "~64 sites / Linux XDG+compositor included / grep env::var -> 0"
was unsafe as written. Two classes are deliberately KEPT as live reads and documented in config.rs:

  * Runtime-mutated session vars. vdisplay::apply_session_env REWRITES the process env on every
    connect (the Bazzite Gaming<->Desktop follow): WAYLAND_DISPLAY, XDG_CURRENT_DESKTOP,
    XDG_RUNTIME_DIR, DBUS_SESSION_BUS_ADDRESS, and the derived PUNKTFUNK_INPUT_BACKEND,
    GAMESCOPE_SESSION/NODE, KWIN/MUTTER_VIRTUAL_PRIMARY, FORCE_SHM. Parsing these once would
    freeze them at startup and silently break session-following — they are NOT constant.
  * Single-use local tuning with no resolve-once benefit (and FEC_PCT even has two different
    semantics): FEC_PCT, VIDEO_DROP, VBV_FRAMES, SPLIT_ENCODE, PACE_BURST_KB, the dxgi timing
    knobs, the *_LIVE/test gates, plus path/dynamic reads (config-dir, PATH search,
    env-forward-to-child). PUNKTFUNK_ZEROCOPY is split on purpose: Windows presence-semantics
    moved to the field; Linux keeps its own truthy (1|true|yes|on) parser.

Verified: Linux cargo check + clippy (-D warnings) + fmt clean on the touched files. The
Windows-only edits are 1:1 substitutions; they get a real Windows compile on the box with Stage 3.

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

crates/punktfunk-host/src/inject.rs

@@ -112,8 +112,10 @@ pub fn default_backend() -> Backend {
     }
     #[cfg(not(target_os = "windows"))]
     {
-        if std::env::var("PUNKTFUNK_COMPOSITOR")
-            .is_ok_and(|v| v.trim().eq_ignore_ascii_case("gamescope"))
+        if crate::config::config()
+            .compositor
+            .as_deref()
+            .is_some_and(|v| v.trim().eq_ignore_ascii_case("gamescope"))
         {
             return Backend::GamescopeEi;
         }
@@ -260,8 +262,10 @@ fn coalesce(events: Vec<InputEvent>) -> Vec<InputEvent> {
 /// (`org.gnome.Mutter.RemoteDesktop`), the same direct API the Mutter video backend uses.
 #[cfg(target_os = "linux")]
 fn libei_ei_source() -> libei::EiSource {
-    let gnome = std::env::var("PUNKTFUNK_COMPOSITOR")
-        .is_ok_and(|v| v.trim().eq_ignore_ascii_case("mutter"))
+    let gnome = crate::config::config()
+        .compositor
+        .as_deref()
+        .is_some_and(|v| v.trim().eq_ignore_ascii_case("mutter"))
         || std::env::var("XDG_CURRENT_DESKTOP")
             .unwrap_or_default()
             .to_ascii_uppercase()