fix(pm): ensure cache dir is on same drive as project on Windows

Hardlinks cannot cross drive boundaries on Windows. When cache is on
C: but project is on D:, utoo falls back to copy (very slow).

Detect cross-drive and use a cache dir on the project's drive instead
(e.g. D:\.cache\nm).

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

Author: elrrrrrrr

crates/pm/src/util/platform_const.rs

@@ -1,4 +1,6 @@
-//! Platform-specific constants for cross-platform compatibility.
+//! Platform-specific constants and utilities for cross-platform compatibility.
+
+use std::path::{Path, PathBuf};
 
 /// PATH environment variable separator.
 /// Windows uses `;`, Unix uses `:`.
@@ -8,3 +10,39 @@ pub const PATH_SEPARATOR: &str = if cfg!(windows) { ";" } else { ":" };
 /// Windows: `<prefix>/node_modules/<pkg>`
 /// Unix: `<prefix>/lib/node_modules/<pkg>`
 pub const GLOBAL_NODE_MODULES: &str = if cfg!(windows) { "node_modules" } else { "lib/node_modules" };
+
+/// Ensure cache directory is on the same drive as the project directory.
+///
+/// On Windows, hardlinks cannot cross drive boundaries (e.g. cache on C:,
+/// project on D:). When a cross-drive situation is detected, returns a
+/// cache path on the project's drive instead.
+///
+/// On Unix, returns the original cache path unchanged.
+pub fn ensure_same_drive(cache_dir: &Path, project_dir: &Path) -> PathBuf {
+    #[cfg(windows)]
+    {
+        let cache_prefix = cache_dir
+            .components()
+            .next()
+            .map(|c| c.as_os_str().to_ascii_uppercase());
+        let project_prefix = project_dir
+            .components()
+            .next()
+            .map(|c| c.as_os_str().to_ascii_uppercase());
+
+        if cache_prefix != project_prefix {
+            if let Some(project_root) = project_dir.components().next() {
+                let alt_cache = PathBuf::from(project_root.as_os_str()).join(".cache").join("nm");
+                tracing::debug!(
+                    "Cache dir {} is on a different drive than project {}, using {} instead",
+                    cache_dir.display(),
+                    project_dir.display(),
+                    alt_cache.display()
+                );
+                return alt_cache;
+            }
+        }
+    }
+    let _ = project_dir; // suppress unused warning on non-windows
+    cache_dir.to_path_buf()
+}

crates/pm/src/util/user_config.rs

@@ -140,7 +140,14 @@ pub async fn set_cache_dir(cache_dir: Option<String>) {
 }
 
 pub fn get_cache_dir() -> PathBuf {
-    PathBuf::from(CACHE_DIR.get_sync())
+    let cache = PathBuf::from(CACHE_DIR.get_sync());
+    // On Windows, ensure cache is on the same drive as the project
+    // to allow hardlinks (which can't cross drive boundaries)
+    if let Ok(cwd) = std::env::current_dir() {
+        super::platform_const::ensure_same_drive(&cache, &cwd)
+    } else {
+        cache
+    }
 }
 
 // Package.json cache — keyed by directory path, covers root + workspace members.