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/crypto/crypto-random_hex.md

@@ -0,0 +1,26 @@
+---
+id: crypto-random_hex
+name: Crypto.random_hex
+category: crypto
+kind: namespace-method
+tokens: Crypto.random_hex
+sig: Crypto.random_hex(n) -> string
+tip: Alias for random_bytes — n secure bytes as a 2n-char hex string.
+order: 7
+ns: Crypto
+member: random_hex
+---
+
+Identical to <a href="crypto-random_bytes"><code>Crypto.random_bytes</code></a>: draws <code>n</code> bytes from the OS CSPRNG and returns them as a <code>2n</code>-character lowercase hex string. The two names are interchangeable; <code>random_hex</code> exists so call sites can be explicit that the return value is already hex text (not raw bytes) when that reads more clearly. The same determinism caveat applies — the result is unpredictable by design and must stay out of the reproducible simulation RNG.
+
+Parameters:
+- `n` — the number of secure random bytes to draw
+
+```ludic
+program Nonce {
+  entry {
+    let nonce = Crypto.random_hex(12)   # 24 hex chars
+    print(len(nonce))                    # 24
+  }
+}
+```