feat(windows-host): HostConfig foundation + staged Goal-1 roadmap (Goal-1 stage 1)

config.rs: typed HostConfig parsed ONCE from env (idd_push/encoder_pref/no_helper/force_helper), replacing per-call env::var re-reads (PUNKTFUNK_ENCODER was re-read on EVERY windows_resolved_backend() call; PUNKTFUNK_IDD_PUSH is read 8x across the host — the recompute that lets capture + encode disagree on the backend, plan §2.4). Migrated the two highest-churn dispatch reads onto it (encode::windows_resolved_backend, punktfunk1::should_use_helper). Behavior-identical: the env is constant for the process lifetime (the service loads host.env before launch), so a lazily-parsed global == parsed-once-at-startup.

docs/windows-host-goal1-plan.md: the ORDERED, independently-shippable execution plan for Goal-1 (the plan's biggest unstarted goal — a from-scratch layered host architecture). Six behavior-preserving, box-verified stages (HostConfig -> SessionPlan -> SessionContext/SessionFactory -> seam-trait tightenings -> src/windows tree), because the host is live-validated and a monolithic rewrite would strand it broken. Stage 1 done here; stages 3-5 rewire the deployed path and require on-glass re-test.

Verified: Linux + box (--features nvenc) cargo check clean.

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

crates/punktfunk-host/src/encode.rs

@@ -450,10 +450,8 @@ enum GpuVendor {
 /// vendor). Shared by [`open_video`] and the GameStream codec advertisement so both agree.
 #[cfg(target_os = "windows")]
 pub(crate) fn windows_resolved_backend() -> WindowsBackend {
-    let pref = std::env::var("PUNKTFUNK_ENCODER")
-        .unwrap_or_default()
-        .to_ascii_lowercase();
-    match pref.as_str() {
+    // Resolved ONCE in HostConfig (Goal-1) — was re-read from PUNKTFUNK_ENCODER on every call.
+    match crate::config::config().encoder_pref.as_str() {
         "nvenc" | "hw" | "nvidia" | "cuda" => WindowsBackend::Nvenc,
         "amf" | "amd" => WindowsBackend::Amf,
         "qsv" | "intel" => WindowsBackend::Qsv,