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-nil.md

@@ -0,0 +1,23 @@
+---
+id: uuid-nil
+name: Uuid.nil
+category: uuid
+kind: namespace-method
+tokens: Uuid.nil
+sig: Uuid.nil() -> string
+tip: The all-zero UUID.
+order: 9
+ns: Uuid
+member: nil
+---
+
+Returns the nil UUID — <code>00000000-0000-0000-0000-000000000000</code> — the reserved all-zero value that means "no UUID". Use it as a sentinel for an unset or absent identity, and as the value <a href="uuid-parse"><code>Uuid.parse</code></a> returns when it is handed something that is not a valid UUID. It is a well-formed UUID string, so it passes <a href="uuid-is_valid"><code>Uuid.is_valid</code></a>; test for "no id" by comparing against <code>Uuid.nil()</code> explicitly.
+
+```ludic
+program Nil {
+  entry {
+    let none = Uuid.nil()
+    print(none)   # 00000000-0000-0000-0000-000000000000
+  }
+}
+```