refactor: simplify cross-drive cache dir detection

Use cwd.ancestors().last() to get drive root instead of manual
component extraction.

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

Author: elrrrrrrr

crates/pm/src/util/platform_const.rs

@@ -1,5 +1,8 @@
 //! Platform-specific constants for cross-platform compatibility.
 
+use std::ffi::OsString;
+use std::path::Path;
+
 /// PATH environment variable separator.
 /// Windows uses `;`, Unix uses `:`.
 pub const PATH_SEPARATOR: &str = if cfg!(windows) { ";" } else { ":" };
@@ -12,3 +15,11 @@ pub const GLOBAL_NODE_MODULES: &str = if cfg!(windows) {
 } else {
     "lib/node_modules"
 };
+
+/// Extract the drive root component (e.g. `C:`) from a path, uppercased for comparison.
+/// Returns `None` on Unix or if the path has no components.
+pub fn drive_root(p: &Path) -> Option<OsString> {
+    p.components()
+        .next()
+        .map(|c| c.as_os_str().to_ascii_uppercase())
+}

crates/pm/src/util/user_config.rs

@@ -1,7 +1,10 @@
 use std::collections::HashSet;
+use std::env;
 use std::path::{Path, PathBuf};
 use std::sync::{LazyLock, OnceLock};
 
+use super::platform_const::drive_root;
+
 use anyhow::Result;
 use dashmap::DashMap;
 use utoo_ruborist::builder::PeerDeps;
@@ -142,20 +145,12 @@ pub async fn set_cache_dir(cache_dir: Option<String>) {
 pub fn get_cache_dir() -> PathBuf {
     let cache = PathBuf::from(CACHE_DIR.get_sync());
 
-    // On Windows, hardlinks cannot cross drive boundaries.
-    // If cache (e.g. C:\.cache\nm) is on a different drive than cwd (e.g. D:\project),
-    // use a cache dir on the project's drive instead.
+    // On Windows, hardlinks cannot cross drive boundaries (e.g. cache on C:, project on D:).
+    // Fall back to a cache dir on the project's drive.
     #[cfg(windows)]
-    if let Ok(cwd) = std::env::current_dir() {
-        fn drive_prefix(p: &Path) -> Option<std::ffi::OsString> {
-            p.components()
-                .next()
-                .map(|c| c.as_os_str().to_ascii_uppercase())
-        }
-        if drive_prefix(&cache) != drive_prefix(&cwd) {
-            if let Some(drive) = cwd.components().next() {
-                return PathBuf::from(drive.as_os_str()).join(".cache").join("nm");
-            }
+    if let Ok(cwd) = env::current_dir() {
+        if drive_root(&cache) != drive_root(&cwd) {
+            return cwd.ancestors().last().unwrap_or(&cwd).join(".cache/nm");
         }
     }