fix: clear() treats a concurrent delete as success; explicit utf-8; durability prose

clear() handles FileNotFoundError as success (the goal state — no file — is reached)
instead of returning False. read/write open with explicit encoding='utf-8'. atomic-write
prose scoped to process-crash safety (NOT power-loss durability — no fsync), in module,
README, and CLAUDE.

Signed-off-by: disqualifier <dev@disqualifier.me>

README.md

@@ -62,8 +62,11 @@ Prefer `AioKV` in new code.
 ## Durability
 
 Writes are **atomic**: data is written to a temp file in the same directory and
-`os.replace()`d over the target (atomic on POSIX). A crash mid-write leaves the
-previous good file intact, and a reader never observes a partial file. A single
+`os.replace()`d over the target (atomic on POSIX). A **process** crash mid-write leaves
+the previous good file intact, and a reader never observes a partial file. (This is
+process-crash safety, not power-loss durability — there's no `fsync`, so an OS/power
+failure could still lose the last write; fine for reconstructible single-process state.)
+A single
 `asyncio.Lock` guards every read and write, so concurrent operations on one instance
 are consistent and no update is lost. All blocking filesystem calls run via
 `asyncio.to_thread`, so nothing stalls the event loop.