Cross-platform persistent save directory (IDBFS on WASM, filesystem on desktop)

Game code uses standard Python file I/O to mcrfpy.save_dir with no platform
branching. On WASM, builtins.open() is monkeypatched so writes to /save/
auto-sync IDBFS on close, making persistence transparent.

API: mcrfpy.save_dir (str), mcrfpy._sync_storage() (auto-called on WASM)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/unit/save_dir_test.py

@@ -0,0 +1,81 @@
+"""Test mcrfpy.save_dir and mcrfpy._sync_storage() persistence API"""
+import mcrfpy
+import os
+import json
+import sys
+
+# Test 1: save_dir attribute exists and is a string
+save_dir = mcrfpy.save_dir
+assert isinstance(save_dir, str), f"save_dir should be str, got {type(save_dir)}"
+print(f"PASS: mcrfpy.save_dir = '{save_dir}'")
+
+# Test 2: save directory exists on disk
+assert os.path.isdir(save_dir), f"save_dir '{save_dir}' does not exist as a directory"
+print(f"PASS: save directory exists")
+
+# Test 3: _sync_storage is callable and returns None
+result = mcrfpy._sync_storage()
+assert result is None, f"_sync_storage should return None, got {result}"
+print(f"PASS: _sync_storage() returns None")
+
+# Test 4: Can write a file to save_dir with context manager
+test_path = os.path.join(save_dir, "test_persistence.json")
+test_data = {"lore_flags": {"sundering": True}, "zone": 3, "enemies": [1, 2, 3]}
+with open(test_path, 'w') as f:
+    json.dump(test_data, f)
+print(f"PASS: wrote {test_path}")
+
+# Test 5: Can read it back
+with open(test_path, 'r') as f:
+    loaded = json.load(f)
+assert loaded == test_data, f"Data mismatch: {loaded} != {test_data}"
+print(f"PASS: read back matches")
+
+# Test 6: Can write binary data
+bin_path = os.path.join(save_dir, "test_binary.dat")
+with open(bin_path, 'wb') as f:
+    f.write(b'\x00\x01\x02\xff' * 100)
+with open(bin_path, 'rb') as f:
+    data = f.read()
+assert len(data) == 400
+assert data[:4] == b'\x00\x01\x02\xff'
+print(f"PASS: binary read/write works")
+
+# Test 7: sync after writes (no-op on desktop, but should not error)
+mcrfpy._sync_storage()
+print(f"PASS: _sync_storage() after writes")
+
+# Test 8: open() on non-save paths is not affected
+# (On WASM, the monkeypatch only wraps writes to /save/)
+import tempfile
+tmp = os.path.join(save_dir, "test_unwrapped.txt")
+with open(tmp, 'r+' if os.path.exists(tmp) else 'w') as f:
+    f.write("test")
+os.remove(tmp)
+print(f"PASS: open() works for other paths")
+
+# Test 9: Verify the cross-platform API contract
+# Game code that works identically on desktop and WASM:
+game_state_path = os.path.join(mcrfpy.save_dir, "game_state.json")
+game_state = {
+    "player": {"hp": 50, "zone": 3},
+    "lore": {"sundering": True, "librarian_met": False},
+    "enemies": {"zone_3": {"goblin_chief": False}}
+}
+# Write (on WASM, context manager auto-syncs IDBFS; on desktop, just writes)
+with open(game_state_path, 'w') as f:
+    json.dump(game_state, f)
+# Read
+with open(game_state_path, 'r') as f:
+    loaded_state = json.load(f)
+assert loaded_state == game_state
+os.remove(game_state_path)
+print(f"PASS: cross-platform save/load contract works")
+
+# Cleanup
+os.remove(test_path)
+os.remove(bin_path)
+print(f"PASS: cleanup done")
+
+print("\nAll save_dir tests passed!")
+sys.exit(0)