fix: str-only keys, symlink-safe atomic writes, strict JSON, stale-tmp sweep

_load() masked permission/IO errors as an empty store via os.path.exists();
now raises FileNotFoundError only, propagating real failures. Non-str keys
silently never round-tripped through get() since JSON object keys are always
strings; set()/get() now raise ValueError on a non-str key. _save() clobbered
a symlinked store file with os.replace(); now realpaths the target first.
json.dumps() allows NaN/Infinity by default, producing invalid JSON for
strict readers; now allow_nan=False. A whitespace-only file raised while a
zero-byte file returned {}; both now return {}. Orphaned .<pid>.tmp files
from a hard crash of a dead process are swept on save. JSON (de)serialization
now runs via asyncio.to_thread instead of on the event loop.

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

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "aiokv"
-version = "0.1.1"
+version = "0.2.0"
 description = "Async file-backed key-value store for single-process local state — atomic writes, no TTL, config-free, installable."
 requires-python = ">=3.10"
 dependencies = [