perf(paths): resolve the library root once, not on every path check (#966)

`Path.resolve()` is a filesystem call — it lstats every component of the path.
`_resolve_dlc_path` and `safe_join` both re-resolved their ROOT on every single
call, and those run once per song, per art fetch, per scanned row.

Found while profiling a 2-fps report: on a real 50,944-song library the server
was issuing ~23,500 stat/lstat calls per second, re-walking the same three
parent directories over and over, and burning ~50% of a core doing it. It is
worst exactly where big libraries live — the library was on an NTFS-3G (FUSE)
mount, where every stat is a userspace round trip through mount.ntfs-3g (itself
visible in top). The cost was the constant re-resolution, not the work.

A root is fixed for the life of the process, so resolve it once
(safepath.resolved_root, lru_cache). Measured, 5,000 lookups against a real
library path:

    before:  15,264 stat syscalls   (54.2 ms)
    after:       277 stat syscalls   ( 0.8 ms)     55x fewer

Containment is unchanged, which is the part that matters:
  - safe_join still resolves the CANDIDATE on every call — following its
    symlinks IS the zip-slip / traversal defence, so it is never cached. Only
    the server-owned root is.
  - _resolve_dlc_path keeps its lexical containment check (deliberately does not
    follow symlinks, so junction-mounted libraries keep working).

Tradeoff, documented on resolved_root: if a root's symlink is re-pointed at a
NEW target while the server runs, the old target stays in effect until restart.
Fine for a library path fixed at startup; the cache is keyed on the Path, so
switching library dir is a different key.

Tests: root resolved once across 500 lookups (the regression), a different root
is a different entry, and the containment contract re-pinned — traversal,
Windows drive-absolute, backslash, NUL, empty, and a symlink escaping the root
must still be refused. Full suite green.

lib/safepath.py

@@ -4,9 +4,34 @@ under a server-owned root.
 
 from __future__ import annotations
 
+from functools import lru_cache
 from pathlib import Path
 
 
+@lru_cache(maxsize=16)
+def resolved_root(root: Path) -> Path:
+    """Canonical (link-resolved) form of a server-owned root directory.
+
+    ``Path.resolve()`` is a filesystem call: it lstats every component of the
+    path. The roots we join against — the DLC library, a plugin's asset dir —
+    are fixed for the life of the process, but the containment helpers below
+    (and ``dlc_paths._resolve_dlc_path``) were re-resolving them on EVERY call,
+    and those are called once per song, per art fetch, per scanned row.
+
+    On a real 50,944-song library that cost ~23,500 stat/lstat calls per second,
+    pinning a core. It is brutal when the library lives on a FUSE mount
+    (NTFS-3G, SMB, sshfs), where every stat is a userspace round trip: the same
+    three parent directories were being walked over and over.
+
+    Cached because a root is a constant here, not because resolution is cheap.
+    Consequence: if a root's symlink/junction is re-pointed at a NEW target
+    while the server is running, the old target stays in effect until restart.
+    That is fine for a library path fixed at startup, and the cache is keyed on
+    the Path, so switching to a different library dir is a different key.
+    """
+    return root.resolve()
+
+
 def safe_join(root: Path, name: str) -> Path | None:
     """Resolve ``name`` under ``root`` and return the resolved Path, or
     ``None`` if it would escape ``root`` or is unrepresentable.
@@ -35,9 +60,12 @@ def safe_join(root: Path, name: str) -> Path | None:
         return None
     safe = name.replace("\\", "/")
     try:
-        root_resolved = root.resolve()
-        candidate = (root_resolved / safe).resolve()
-        if not candidate.is_relative_to(root_resolved):
+        # The ROOT is a constant — resolve it once (see resolved_root). The
+        # CANDIDATE must still be resolved on every call: following its symlinks
+        # is exactly the zip-slip / traversal defence, so it is never cached.
+        root_res = resolved_root(root)
+        candidate = (root_res / safe).resolve()
+        if not candidate.is_relative_to(root_res):
             return None
     except (ValueError, OSError):
         return None