feat(stdlib): finish Crypto (CSPRNG + base64) and add Uuid.* library (#19 #16)

Crypto (#19): add the OS cryptographically-secure random surface
(random_bytes/random_hex/random_u32, reading /dev/urandom) and a standard
base64 encoder, completing the library alongside the existing SHA-256/
HMAC-SHA256/verify_hmac/hex/ct_equal. All pure integer IR, C-free.

Uuid (#16): a new namespace for stable, collision-free IDs — v4 (random) and
v7 (time-ordered) generation, plus parse/is_valid/to_text/equals/nil. UUIDs are
canonical lowercase 36-char strings; v4 and v7's random tail draw from the
crypto CSPRNG, so both carry the documented determinism caveat (mint at the
edges, never inside lockstep simulation). Reuses the crypto prelude's
fn_secure_bytes / fn_hex_encode.

- examples/library/{crypto,uuid}.ludic: known-answer vectors (SHA-256, HMAC,
  base64 per RFC 4231/4648) and structural invariants (uuid version/variant
  bits, parse/equals), wired into `x test` (now 51 passed).
- docs: per-symbol pages for every new method + a new Uuid section; inventory
  and impl-vs-docs coverage check pass.
- seed regenerated; `x bootstrap-cfree` fixpoint holds.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/language/uuid/uuid-new.md

@@ -0,0 +1,25 @@
+---
+id: uuid-new
+name: Uuid.new
+category: uuid
+kind: namespace-method
+tokens: Uuid.new
+sig: Uuid.new() -> string
+tip: A new random (v4) UUID as a canonical 36-char string.
+order: 1
+ns: Uuid
+member: new
+---
+
+Generates a new **v4** (random) UUID and returns it in canonical lowercase text form, e.g. <code>550e8400-e29b-41d4-a716-446655440000</code>. A v4 UUID carries 122 bits of entropy drawn from the OS secure random generator, so two independently-generated IDs colliding is not something you will ever observe in practice — which is exactly what makes it a good identity for a player, a session, a networked entity, or a piece of user-created content that has no central authority to number it.
+
+This is the default UUID constructor; <a href="uuid-v4"><code>Uuid.v4</code></a> is an explicit alias. Because the value is random it is **non-deterministic** — do not mint UUIDs inside the lockstep simulation, or replays and peers will diverge.
+
+```ludic
+program NewId {
+  entry {
+    let id = Uuid.new()
+    print(len(id))   # 36
+  }
+}
+```