random(): per-thread singleton; random_key/random_nonce use it

Each cipher module's random() now returns a thread-local Random that is
created on first use and reused thereafter. Each thread gets its own
OS-seeded generator (own key+nonce), so no locking and no cross-thread
nonce collisions. random_key() and random_nonce() draw from this CSPRNG
instead of secrets.token_bytes, making them as fast as the stream.
Changes made in the aegis256x4.py template and regenerated.

src/aeg/random.py

@@ -31,8 +31,9 @@ class Random:
     Each call produces keystream under the current key/nonce and increments
     the nonce afterwards, so no output block is ever produced twice.
 
-    The primary way to create one is the cipher module's own random()
-    helper, e.g. ``aeg.cipher("AEGIS-128X2").random()``.
+    The primary way to get one is the cipher module's own random() helper,
+    e.g. ``aeg.cipher("AEGIS-128X2").random()``, which returns a per-thread
+    singleton (each thread gets its own OS-seeded generator; no locking).
     """
 
     def __init__(self, cipher, *, seed: Buffer | None = None):