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_write.py

@@ -0,0 +1,161 @@
+# -*- coding: utf-8 -*-
+from __future__ import print_function, unicode_literals
+
+import os
+import unittest
+from io import BytesIO
+
+from conftest import BeetFSTestCase, is_ffmpeg_available, is_flac_available
+
+
+@unittest.skipUnless(is_ffmpeg_available() and is_flac_available(),
+                     "ffmpeg and flac required for write tests")
+class TestWriteMetadata(BeetFSTestCase):
+    
+    def setUp(self):
+        super(TestWriteMetadata, self).setUp()
+        try:
+            import mutagen.flac
+            self.mutagen = mutagen.flac
+        except ImportError:
+            self.mutagen = None
+    
+    def test_write_opens_without_error(self):
+        self.add_test_track(filename='write_test.flac')
+        self.mount_beetfs()
+        
+        tracks = self._find_any_track()
+        if not tracks:
+            self.skipTest("No tracks found")
+        
+        try:
+            with open(tracks[0], 'r+b') as f:
+                pass
+        except IOError:
+            self.skipTest("Write not supported")
+    
+    def test_write_to_header_updates_db(self):
+        if not self.mutagen:
+            self.skipTest("mutagen required")
+        
+        file_path, item_id = self.add_test_track(
+            filename='db_write_test.flac',
+            artist='Original Artist',
+            title='Original Title'
+        )
+        
+        self.mount_beetfs()
+        
+        tracks = self._find_any_track()
+        if not tracks:
+            self.skipTest("No tracks found")
+        
+        with open(tracks[0], 'rb') as f:
+            original_data = f.read()
+        
+        flac = self.mutagen.FLAC(BytesIO(original_data))
+        flac['artist'] = ['Modified Artist']
+        
+        output = BytesIO()
+        flac.save(output)
+        modified_data = output.getvalue()
+        
+        try:
+            with open(tracks[0], 'r+b') as f:
+                f.write(modified_data[:1024])
+        except IOError:
+            self.skipTest("Write not supported")
+        
+        item = self.library.get_item(item_id)
+        if item:
+            self.assertIn('Modified', str(item))
+    
+    def test_original_file_unchanged_after_write(self):
+        if not self.mutagen:
+            self.skipTest("mutagen required")
+        
+        file_path, item_id = self.add_test_track(
+            filename='unchanged_test.flac',
+            artist='Original Artist'
+        )
+        
+        with open(file_path, 'rb') as f:
+            original_content = f.read()
+        
+        self.mount_beetfs()
+        
+        tracks = self._find_any_track()
+        if tracks:
+            try:
+                with open(tracks[0], 'r+b') as f:
+                    f.write(b'\x00' * 100)
+            except IOError:
+                pass
+        
+        with open(file_path, 'rb') as f:
+            after_content = f.read()
+        
+        self.assertEqual(original_content, after_content)
+    
+    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 TestWriteAudioDiscarded(BeetFSTestCase):
+    
+    def test_write_to_audio_region_no_error(self):
+        file_path, item_id = self.add_test_track(filename='audio_write.flac')
+        
+        self.mount_beetfs()
+        
+        tracks = self._find_any_track()
+        if not tracks:
+            self.skipTest("No tracks found")
+        
+        try:
+            with open(tracks[0], 'r+b') as f:
+                f.seek(10000)
+                f.write(b'\x00' * 100)
+        except IOError:
+            pass
+    
+    def test_audio_unchanged_after_write_attempt(self):
+        file_path, item_id = self.add_test_track(filename='audio_unchanged.flac')
+        
+        with open(file_path, 'rb') as f:
+            f.seek(10000)
+            original_audio = f.read(100)
+        
+        self.mount_beetfs()
+        
+        tracks = self._find_any_track()
+        if tracks:
+            try:
+                with open(tracks[0], 'r+b') as f:
+                    f.seek(10000)
+                    f.write(b'\xff' * 100)
+            except IOError:
+                pass
+        
+        with open(file_path, 'rb') as f:
+            f.seek(10000)
+            after_audio = f.read(100)
+        
+        self.assertEqual(original_audio, after_audio)
+    
+    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
+
+
+if __name__ == '__main__':
+    unittest.main()