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/config.rs

@@ -0,0 +1,45 @@
+//! `HostConfig` — the host's runtime knobs parsed ONCE from the environment, instead of the ~68 scattered
+//! `env::var` reads recomputed at every call site (some up to 8×, which lets capture + encode silently
+//! disagree on the resolved backend — plan §2.4). The service / launcher loads `host.env` into the process
+//! environment before the host starts, and the environment is constant for the process lifetime, so a
+//! lazily-parsed global is equivalent to "parsed once at startup".
+//!
+//! **Goal-1 stage 1** (`docs/windows-host-goal1-plan.md`): this is the foundation. Subsequent stages grow
+//! this struct + migrate the remaining read sites onto it, then `SessionPlan` (stage 2) consumes it as the
+//! single owner of the capture/topology/encoder decision. New fields are added here AS call sites migrate —
+//! a field that nothing reads yet would just be dead, so they land together with their migration.
+
+use std::sync::OnceLock;
+
+/// Resolved host configuration. Grows as `env::var` call sites migrate onto it (Goal-1).
+#[derive(Debug, Clone, Default)]
+pub struct HostConfig {
+    /// `PUNKTFUNK_IDD_PUSH` — use the IDD direct-push capturer (in-process Session-0 capture; no WGC helper).
+    pub idd_push: bool,
+    /// `PUNKTFUNK_ENCODER` — explicit encoder-backend override (lowercased; empty = auto-detect by GPU vendor).
+    pub encoder_pref: String,
+    /// `PUNKTFUNK_NO_HELPER` — never spawn the user-session WGC helper.
+    pub no_helper: bool,
+    /// `PUNKTFUNK_FORCE_HELPER` — force the WGC helper even when not running as SYSTEM.
+    pub force_helper: bool,
+}
+
+impl HostConfig {
+    fn from_env() -> Self {
+        let flag = |k: &str| std::env::var_os(k).is_some();
+        Self {
+            idd_push: flag("PUNKTFUNK_IDD_PUSH"),
+            encoder_pref: std::env::var("PUNKTFUNK_ENCODER")
+                .unwrap_or_default()
+                .to_ascii_lowercase(),
+            no_helper: flag("PUNKTFUNK_NO_HELPER"),
+            force_helper: flag("PUNKTFUNK_FORCE_HELPER"),
+        }
+    }
+}
+
+/// The process-wide host configuration, parsed once on first access.
+pub fn config() -> &'static HostConfig {
+    static CFG: OnceLock<HostConfig> = OnceLock::new();
+    CFG.get_or_init(HostConfig::from_env)
+}