Set up Go module and move Rust to legacy

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

legacy/src/window/mod.rs

@@ -0,0 +1,45 @@
+use crate::domain::EvidenceHealth;
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct WindowSnapshot {
+    pub title: String,
+    pub class: Option<String>,
+    pub health: EvidenceHealth,
+}
+
+impl WindowSnapshot {
+    pub fn unavailable(reason: impl Into<String>) -> Self {
+        Self {
+            title: "none".to_string(),
+            class: None,
+            health: EvidenceHealth::Unavailable(reason.into()),
+        }
+    }
+}
+
+#[cfg(target_os = "windows")]
+mod windows;
+#[cfg(target_os = "windows")]
+pub use windows::*;
+
+#[cfg(target_os = "linux")]
+mod linux;
+#[cfg(target_os = "linux")]
+pub use linux::*;
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn unavailable_snapshot_records_reason_without_evidence() {
+        assert_eq!(
+            WindowSnapshot::unavailable("no active window"),
+            WindowSnapshot {
+                title: "none".to_string(),
+                class: None,
+                health: EvidenceHealth::Unavailable("no active window".to_string()),
+            }
+        );
+    }
+}