fix: delay-load wpcap.dll so binary starts without Npcap installed（build publish, pypi publish, crates publish）

Previously the npcap feature (enabled by default) caused wpcap.dll to
be a load-time dependency. On Windows machines without Npcap, the OS
loader terminated the process immediately  no output, no error, even
--help and --version were silent.

Changes:
- build.rs: add /DELAYLOAD:wpcap.dll for Windows MSVC targets
- loopback.rs: pre-check wpcap.dll with LoadLibraryA before calling
  pcap APIs, return friendly error instead of crashing
- bump version to 0.1.7-rc.12

Author: VincentZyuwin11台式机

py/pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "winload"
-version = "0.1.7-rc.11"
+version = "0.1.7-rc.12"
 description = "Network Load Monitor - nload-like TUI tool for Windows"
 readme = "readme.md"
 license = { text = "MIT" }

rust/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "winload"
-version = "0.1.7-rc.11"
+version = "0.1.7-rc.12"
 edition = "2021"
 description = "Network Load Monitor — nload-like TUI tool for Windows/Linux/macOS"
 license = "MIT"

rust/build.rs

@@ -1,3 +1,14 @@
 fn main() {
-    println!("cargo:rustc-env=TARGET={}", std::env::var("TARGET").unwrap());
+    let target = std::env::var("TARGET").unwrap();
+    println!("cargo:rustc-env=TARGET={target}");
+
+    // Windows + npcap feature: delay-load wpcap.dll
+    // Without this, the binary fails to start on machines without Npcap installed,
+    // because the OS loader cannot find wpcap.dll at process startup.
+    // With /DELAYLOAD, wpcap.dll is only loaded when pcap functions are first called
+    // (i.e., only when --npcap flag is used at runtime).
+    if target.contains("windows") && cfg!(feature = "npcap") {
+        println!("cargo:rustc-link-lib=delayimp");
+        println!("cargo:rustc-link-arg=/DELAYLOAD:wpcap.dll");
+    }
 }

rust/src/loopback.rs

@@ -53,12 +53,45 @@ pub mod platform {
     use super::*;
     use std::thread;
 
+    /// Check if wpcap.dll (Npcap) is available on this system.
+    /// Must be called before any pcap API calls when using /DELAYLOAD,
+    /// otherwise the delay-load helper will raise a fatal exception.
+    #[cfg(feature = "npcap")]
+    fn is_npcap_installed() -> bool {
+        extern "system" {
+            fn LoadLibraryA(name: *const u8) -> *mut std::ffi::c_void;
+            fn FreeLibrary(handle: *mut std::ffi::c_void) -> i32;
+        }
+        unsafe {
+            let handle = LoadLibraryA(b"wpcap.dll\0".as_ptr());
+            if handle.is_null() {
+                false
+            } else {
+                FreeLibrary(handle);
+                true
+            }
+        }
+    }
+
     /// 启动 Npcap 回环捕获线程
     ///
     /// 返回 Ok(info_msg) 成功时，后台线程会持续累加计数器。
     /// 返回 Err(msg) 如果 Npcap 不可用或打开设备失败。
     #[cfg(feature = "npcap")]
     pub fn start_npcap(counters: LoopbackCounters) -> Result<String, String> {
+        // Pre-flight: verify wpcap.dll is loadable before calling any pcap functions.
+        // The binary uses /DELAYLOAD:wpcap.dll, so pcap functions are not resolved
+        // until first call. If the DLL is missing, that first call would crash.
+        if !is_npcap_installed() {
+            return Err(format!(
+                "Npcap is not installed on this system.\n\n\
+                 The --npcap flag requires Npcap to capture loopback traffic.\n\
+                 Please install Npcap from: {NPCAP_URL}\n\
+                 (Enable 'Support loopback traffic capture' during installation)\n\n\
+                 Or run without the --npcap flag."
+            ));
+        }
+
         // 尝试查找 Npcap Loopback 适配器
         let devices = pcap::Device::list().map_err(|e| {
             format!(