perf: load the store once and serve reads from memory

T3-17: every get/set/delete re-read and re-parsed the whole file (get 40 ms,
set 63 ms at 100k keys). the file is now loaded once into memory; reads
return deep copies, set stores the value's json round-trip so memory always
equals a reload, and a failed write rolls the in-memory change back. the
stale-temp sweep runs once per instance. an external edit after first load
is no longer seen (single-process by contract). folds into 1.1.0.

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

README.md

@@ -59,6 +59,16 @@ kv = aiocache("state.json")
 
 Prefer `AioKV` in new code.
 
+## Memory model
+
+The file is read once, on first use, and then served from memory: `get` / `get_all` don't
+touch the disk, and each `set` / `delete` writes the whole store back atomically. `get` and
+`get_all` return deep copies, and `set` stores the value's JSON round-trip (exactly what a
+reload would give back), so mutating a value you passed in or got back never changes the
+store. Because it is single-process, an edit to the file made by something else after the
+first read is not seen (and is overwritten by the next write). Orphaned temp files are swept
+once per instance, on its first save.
+
 ## Durability
 
 Writes are **atomic**: data is written to a temp file in the same directory and