add Windows support (Phase 1)

Cross-platform paths: config_dir() uses %APPDATA%, data_dir() uses
%PROGRAMDATA% on Windows. TLS cert directory uses data_dir() instead
of hardcoded /usr/local/var/numa. Windows DNS discovery via ipconfig.
Fixed cfg gates from not(macos) to explicit linux to prevent Linux
code compiling on Windows. Added Windows target to CI and release
workflows with zip packaging.

System integration (numa install/service) not yet supported on Windows
— users run numa.exe manually.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/system_dns.rs

@@ -24,13 +24,25 @@ pub fn discover_system_dns() -> SystemDnsInfo {
     {
         discover_macos()
     }
-    #[cfg(not(target_os = "macos"))]
+    #[cfg(target_os = "linux")]
     {
         SystemDnsInfo {
             default_upstream: detect_upstream_linux_or_backup(),
             forwarding_rules: Vec::new(),
         }
     }
+    #[cfg(windows)]
+    {
+        discover_windows()
+    }
+    #[cfg(not(any(target_os = "macos", target_os = "linux", windows)))]
+    {
+        log::debug!("no conditional forwarding rules discovered");
+        SystemDnsInfo {
+            default_upstream: None,
+            forwarding_rules: Vec::new(),
+        }
+    }
 }
 
 #[cfg(target_os = "macos")]
@@ -156,7 +168,7 @@ fn make_rule(domain: &str, nameserver: &str) -> Option<ForwardingRule> {
 
 /// Detect upstream from /etc/resolv.conf, falling back to backup file if resolv.conf
 /// only has loopback (meaning numa install already ran).
-#[cfg(not(target_os = "macos"))]
+#[cfg(target_os = "linux")]
 fn detect_upstream_linux_or_backup() -> Option<String> {
     // Try /etc/resolv.conf first
     if let Some(ns) = read_upstream_from_file("/etc/resolv.conf") {
@@ -177,7 +189,7 @@ fn detect_upstream_linux_or_backup() -> Option<String> {
     None
 }
 
-#[cfg(not(target_os = "macos"))]
+#[cfg(target_os = "linux")]
 fn read_upstream_from_file(path: &str) -> Option<String> {
     let text = std::fs::read_to_string(path).ok()?;
     for line in text.lines() {
@@ -193,6 +205,56 @@ fn read_upstream_from_file(path: &str) -> Option<String> {
     None
 }
 
+// --- Windows implementation ---
+
+#[cfg(windows)]
+fn discover_windows() -> SystemDnsInfo {
+    use log::{debug, warn};
+
+    let output = match std::process::Command::new("ipconfig").arg("/all").output() {
+        Ok(o) => o,
+        Err(e) => {
+            warn!("failed to run ipconfig /all: {}", e);
+            return SystemDnsInfo {
+                default_upstream: None,
+                forwarding_rules: Vec::new(),
+            };
+        }
+    };
+
+    let text = String::from_utf8_lossy(&output.stdout);
+    let mut upstream = None;
+
+    for line in text.lines() {
+        let trimmed = line.trim();
+        // Match "DNS Servers" line (English) or similar localized variants
+        if trimmed.contains("DNS Servers") || trimmed.contains("DNS-Server") {
+            if let Some(ip) = trimmed.split(':').last() {
+                let ip = ip.trim();
+                if !ip.is_empty() && ip != "127.0.0.1" && ip != "::1" {
+                    upstream = Some(ip.to_string());
+                    break;
+                }
+            }
+        }
+        // Continuation lines (indented IPs after DNS Servers line)
+        if upstream.is_none() && trimmed.chars().next().map_or(false, |c| c.is_ascii_digit()) {
+            // Skip continuation lines — we only need the first DNS server
+        }
+    }
+
+    if let Some(ref ns) = upstream {
+        info!("detected Windows upstream: {}", ns);
+    } else {
+        debug!("no DNS servers found in ipconfig output");
+    }
+
+    SystemDnsInfo {
+        default_upstream: upstream,
+        forwarding_rules: Vec::new(),
+    }
+}
+
 /// Find the upstream for a domain by checking forwarding rules.
 /// Returns None if no rule matches (use default upstream).
 /// Zero-allocation on the hot path — dot_suffix is pre-computed.
@@ -769,7 +831,7 @@ fn run_systemctl(args: &[&str]) -> Result<(), String> {
 // --- CA trust management ---
 
 fn trust_ca() -> Result<(), String> {
-    let ca_path = std::path::PathBuf::from("/usr/local/var/numa/ca.pem");
+    let ca_path = crate::data_dir().join("ca.pem");
     if !ca_path.exists() {
         return Err("CA not generated yet — start numa first to create certificates".into());
     }
@@ -816,7 +878,7 @@ fn trust_ca() -> Result<(), String> {
 }
 
 fn untrust_ca() -> Result<(), String> {
-    let ca_path = std::path::PathBuf::from("/usr/local/var/numa/ca.pem");
+    let ca_path = crate::data_dir().join("ca.pem");
 
     #[cfg(target_os = "macos")]
     {