feat: parallelize dependency fetching and installs with rayon

Both the resolver and installer now run their per-dependency work in
parallel via rayon::par_iter, so N module clones, N plugin asset
downloads, and N module exports proceed concurrently instead of
serially.

Supporting changes:
- git::clone_or_fetch takes a per-URL mutex so concurrent callers for
  the same repo serialize rather than corrupting the on-disk cache.
- ui::LOG_CAPTURE moved from thread_local to a global Mutex so the TUI
  captures log lines emitted from rayon worker threads.
- Added a global mutex around the cargo-fallback stdin prompt and an
  AtomicU64 counter for staging dir names so concurrent installs can't
  collide on a nanosecond timestamp.

Author: freepicheep

Cargo.lock

@@ -25,6 +25,7 @@ ratatui = "0.29"
 crossterm = "0.29"
 syntect = "5.2"
 tui-scrollview = "0.5"
+rayon = "1"
 
 [dependencies.nu-protocol]
 version = "0.112.2"

Cargo.toml

@@ -1,6 +1,6 @@
-use std::collections::HashSet;
+use std::collections::{HashMap, HashSet};
 use std::path::{Path, PathBuf};
-use std::sync::{Mutex, OnceLock};
+use std::sync::{Arc, Mutex, OnceLock};
 
 use git2::{FetchOptions, Progress, RemoteCallbacks, Repository, build::RepoBuilder};
 use indicatif::ProgressBar;
@@ -11,6 +11,21 @@ use crate::error::{QuiverError, Result};
 use crate::ui;
 
 static FETCHED_THIS_RUN: OnceLock<Mutex<HashSet<String>>> = OnceLock::new();
+static URL_LOCKS: OnceLock<Mutex<HashMap<String, Arc<Mutex<()>>>>> = OnceLock::new();
+
+fn url_locks() -> &'static Mutex<HashMap<String, Arc<Mutex<()>>>> {
+    URL_LOCKS.get_or_init(|| Mutex::new(HashMap::new()))
+}
+
+/// Acquire a per-URL serialization lock so concurrent callers don't race on the
+/// same cached repo. Returns an owned guard that releases on drop.
+fn lock_for_url(url: &str) -> Arc<Mutex<()>> {
+    let key = normalize_git_url(url);
+    let mut map = url_locks().lock().expect("url lock map poisoned");
+    map.entry(key)
+        .or_insert_with(|| Arc::new(Mutex::new(())))
+        .clone()
+}
 
 /// Returns the global install directory for git repos:
 /// `~/.local/share/quiver/installs/git/` on macOS/Linux.
@@ -73,6 +88,11 @@ pub fn clone_or_fetch(url: &str) -> Result<PathBuf> {
     let repo_dir = cache.join(url_to_dirname(url));
     let repo_label = repo_name_from_url(url).unwrap_or_else(|| url.to_string());
 
+    // Serialize concurrent callers for the same URL — git2 operations against a
+    // shared on-disk repo are not safe to overlap.
+    let url_lock = lock_for_url(url);
+    let _url_guard = url_lock.lock().expect("url lock poisoned");
+
     if repo_dir.exists() {
         if was_fetched_this_run(url) {
             return Ok(repo_dir);