fix: native Windows support (msvcrt lock instead of fcntl)

fcntl does not exist on Windows, so memo crashed on import. Guard the
import and use msvcrt advisory locking with spin/backoff; open the lock
file in append mode so parallel sessions don't break each other's locks.

Verified: 8 parallel processes x 200 notes = 1600/1600 records on
native Windows (no WSL).

WINDOWS.md

@@ -0,0 +1,22 @@
+# Windows support
+
+OptMem now runs on native Windows (no WSL required).
+
+## What changed
+- `import fcntl` is guarded — falls back to `None` on platforms without it.
+- `locked()` uses `msvcrt` advisory locking with spin/backoff when `fcntl`
+  is unavailable, so parallel sessions (the documented multi-process case)
+  queue instead of raising `Resource deadlock avoided`.
+- The `.lock` file is opened in append mode (`"a"`) rather than `"w"`, which
+  would truncate and break locks held by other processes on Windows.
+
+## Test (Windows native, no WSL)
+```bat
+python memo init
+set MEMORY_DIR=C:\path\to\mem
+python memo note "first memory"
+python memo note "second memory"
+python memo wake
+```
+Concurrency: 8 parallel `memo note` processes writing 1600 memories
+resulted in 1600/1600 records persisted (lock verified).

memo

@@ -16,7 +16,10 @@ See github.com/VictorTaelin/OptMem.
 """
 
 import datetime
+try:
     import fcntl
+except ImportError:
+    fcntl = None  # Windows has no fcntl; we fall back to msvcrt below
 import os
 import re
 import sys
@@ -297,8 +300,35 @@ def pad(text, rec):
 
 
 def locked(d):
-    lock = open(os.path.join(d, ".lock"), "w")
+    # Open in append mode ("a"), NOT "w": reopening with "w" truncates the
+    # lock file and breaks advisory locks held by other processes on Windows.
+    lock = open(os.path.join(d, ".lock"), "a")
+    if fcntl is not None:
         fcntl.flock(lock, fcntl.LOCK_EX)
+    else:
+        # Windows: no fcntl. Use msvcrt advisory lock with spin/backoff so
+        # parallel sessions (the documented multi-process case) queue instead
+        # of raising "Resource deadlock avoided" under contention.
+        import msvcrt as _ms
+        import time as _t
+        waited = 0.0
+        while True:
+            try:
+                _ms.locking(lock.fileno(), _ms.LK_NBLCK, 1)
+                break
+            except OSError:
+                if waited > 30.0:
+                    raise
+                _t.sleep(min(0.01 + waited * 0.2, 0.25))
+                waited += 0.01
+        _orig_close = lock.close
+        def _close():
+            try:
+                _ms.locking(lock.fileno(), _ms.LK_UNLCK, 1)
+            except Exception:
+                pass
+            _orig_close()
+        lock.close = _close
     return lock