Clear the stranded tmpfs cache at startup

The session map is memory-only, so session directories surviving a restart
can never be served and never be evicted -- and the work root is a tmpfs,
so that is leaked RAM until reboot. The restart that ships the TTFB fix
would have stranded 1.56 GB.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/test_proxy.py

@@ -484,6 +484,32 @@ def test_concurrency_cap(tmp_path):
     assert len(manager.start_log) == 2
 
 
+def test_startup_clears_untracked_cache(tmp_path):
+    """The work root is a tmpfs and the session map is memory-only, so anything
+    left by a previous run is unreachable *and* unevictable -- it would leak RAM
+    until the next reboot."""
+    work = tmp_path / "work"
+    (work / "abcdefghijk").mkdir(parents=True)
+    (work / "abcdefghijk" / "out.mp4").write_bytes(b"x" * 5000)
+    (work / "bcdefghijkl").mkdir()
+    (work / "bcdefghijkl" / "out.mp4").write_bytes(b"y" * 3000)
+    (work / "loose.txt").write_text("not a session")
+
+    reclaimed = proxy.reset_work_root(str(work))
+
+    assert reclaimed == 8000
+    assert not (work / "abcdefghijk").exists()
+    assert not (work / "bcdefghijkl").exists()
+    # A stray file is not a session directory and is left alone.
+    assert (work / "loose.txt").exists()
+
+
+def test_startup_on_a_clean_work_root_is_a_no_op(tmp_path):
+    work = tmp_path / "empty"
+    work.mkdir()
+    assert proxy.reset_work_root(str(work)) == 0
+
+
 def test_no_fetch_mode_refuses_everything(manager_factory):
     manager = manager_factory(no_fetch=True, max_starts=99)
     session, refusal = manager.get(vid(30))