Add e2e test suite for beetfs

Tests use real FUSE operations against mounted beetfs filesystem:
- test_smoke: mount/unmount lifecycle
- test_nested_bug: detects critical indentation bug (13 failures)
- test_readdir: directory listing
- test_read: metadata overlay verification
- test_stat: file/directory attributes
- test_write: metadata modification
- test_error_handling: ENOENT, EOPNOTSUPP

Also includes:
- conftest.py with BeetFSTestCase base class and synthetic FLAC generator
- e2e-test-plan.md with Oracle-reviewed test strategy
- flake.nix updated with ffmpeg/flac for test fixtures

Run: cd tests && nix develop ../ --command python -m unittest discover

tests/test_error_handling.py

@@ -0,0 +1,161 @@
+# -*- coding: utf-8 -*-
+from __future__ import print_function, unicode_literals
+
+import errno
+import os
+import unittest
+
+from conftest import BeetFSTestCase
+
+
+class TestENOENT(BeetFSTestCase):
+    
+    def test_stat_nonexistent_file(self):
+        self.mount_beetfs()
+        
+        nonexistent = os.path.join(self.mount_dir, 'does_not_exist.flac')
+        
+        with self.assertRaises(OSError) as ctx:
+            os.stat(nonexistent)
+        
+        self.assertEqual(ctx.exception.errno, errno.ENOENT)
+    
+    def test_open_nonexistent_file(self):
+        self.mount_beetfs()
+        
+        nonexistent = os.path.join(self.mount_dir, 'does_not_exist.flac')
+        
+        with self.assertRaises(IOError) as ctx:
+            open(nonexistent, 'rb')
+        
+        self.assertEqual(ctx.exception.errno, errno.ENOENT)
+    
+    def test_listdir_nonexistent_directory(self):
+        self.mount_beetfs()
+        
+        nonexistent = os.path.join(self.mount_dir, 'Nonexistent Artist')
+        
+        with self.assertRaises(OSError) as ctx:
+            os.listdir(nonexistent)
+        
+        self.assertEqual(ctx.exception.errno, errno.ENOENT)
+    
+    def test_stat_deeply_nested_nonexistent(self):
+        self.mount_beetfs()
+        
+        nonexistent = os.path.join(
+            self.mount_dir, 'Artist', 'Album', 'Track', 'Nested', 'Path'
+        )
+        
+        with self.assertRaises(OSError) as ctx:
+            os.stat(nonexistent)
+        
+        self.assertEqual(ctx.exception.errno, errno.ENOENT)
+
+
+class TestEOPNOTSUPP(BeetFSTestCase):
+    
+    def test_mkdir_not_supported(self):
+        self.mount_beetfs()
+        
+        new_dir = os.path.join(self.mount_dir, 'new_directory')
+        
+        with self.assertRaises(OSError) as ctx:
+            os.mkdir(new_dir)
+        
+        self.assertIn(ctx.exception.errno, [errno.EOPNOTSUPP, errno.EACCES, errno.EROFS])
+    
+    def test_unlink_not_supported(self):
+        self.add_test_track(filename='delete_test.flac')
+        self.mount_beetfs()
+        
+        tracks = self._find_any_track()
+        if not tracks:
+            self.skipTest("No tracks found")
+        
+        with self.assertRaises(OSError) as ctx:
+            os.unlink(tracks[0])
+        
+        self.assertIn(ctx.exception.errno, [errno.EOPNOTSUPP, errno.EACCES, errno.EROFS])
+    
+    def test_rmdir_not_supported(self):
+        self.add_test_track(filename='rmdir_test.flac', artist='Rmdir Artist')
+        self.mount_beetfs()
+        
+        artist_dir = os.path.join(self.mount_dir, 'Rmdir Artist')
+        if not os.path.isdir(artist_dir):
+            self.skipTest("Artist directory not found")
+        
+        with self.assertRaises(OSError) as ctx:
+            os.rmdir(artist_dir)
+        
+        self.assertIn(ctx.exception.errno, [errno.EOPNOTSUPP, errno.EACCES, errno.EROFS, errno.ENOTEMPTY])
+    
+    def test_rename_not_supported(self):
+        self.add_test_track(filename='rename_test.flac')
+        self.mount_beetfs()
+        
+        tracks = self._find_any_track()
+        if not tracks:
+            self.skipTest("No tracks found")
+        
+        new_path = tracks[0] + '.renamed'
+        
+        with self.assertRaises(OSError) as ctx:
+            os.rename(tracks[0], new_path)
+        
+        self.assertIn(ctx.exception.errno, [errno.EOPNOTSUPP, errno.EACCES, errno.EROFS])
+    
+    def test_symlink_not_supported(self):
+        self.mount_beetfs()
+        
+        link_path = os.path.join(self.mount_dir, 'test_link')
+        
+        with self.assertRaises(OSError) as ctx:
+            os.symlink('/tmp', link_path)
+        
+        self.assertIn(ctx.exception.errno, [errno.EOPNOTSUPP, errno.EACCES, errno.EROFS])
+    
+    def test_chmod_not_supported(self):
+        self.add_test_track(filename='chmod_test.flac')
+        self.mount_beetfs()
+        
+        tracks = self._find_any_track()
+        if not tracks:
+            self.skipTest("No tracks found")
+        
+        with self.assertRaises(OSError) as ctx:
+            os.chmod(tracks[0], 0o777)
+        
+        self.assertIn(ctx.exception.errno, [errno.EOPNOTSUPP, errno.EACCES, errno.EROFS])
+    
+    def _find_any_track(self):
+        results = []
+        for root, dirs, files in os.walk(self.mount_dir):
+            for f in files:
+                if f.endswith('.flac'):
+                    results.append(os.path.join(root, f))
+        return results
+
+
+class TestEdgeCaseErrors(BeetFSTestCase):
+    
+    def test_access_empty_path(self):
+        self.mount_beetfs()
+        
+        with self.assertRaises((OSError, IOError)):
+            os.stat(os.path.join(self.mount_dir, ''))
+    
+    def test_open_directory_as_file(self):
+        self.add_test_track(filename='test.flac', artist='Dir Artist')
+        self.mount_beetfs()
+        
+        artist_dir = os.path.join(self.mount_dir, 'Dir Artist')
+        if os.path.isdir(artist_dir):
+            with self.assertRaises((OSError, IOError)):
+                with open(artist_dir, 'rb') as f:
+                    f.read()
+
+
+if __name__ == '__main__':
+    unittest.main()