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_u32.md

@@ -0,0 +1,26 @@
+---
+id: crypto-random_u32
+name: Crypto.random_u32
+category: crypto
+kind: namespace-method
+tokens: Crypto.random_u32
+sig: Crypto.random_u32() -> int
+tip: One CSPRNG-drawn 32-bit integer.
+order: 8
+ns: Crypto
+member: random_u32
+---
+
+Draws four bytes from the OS CSPRNG and assembles them into one 32-bit integer. Use it when you need a single unpredictable number rather than a hex string — a random challenge value, a per-run identifier, or a non-deterministic seed to hand to a *fresh* <a href="ns-Random"><code>Random</code></a> stream at startup. Because the bytes come from the secure generator, the value prints as a signed integer and can be negative.
+
+Like the other secure-random helpers this is **non-deterministic** and must not be called inside the lockstep simulation: doing so desyncs replays and networked peers. Draw it at the edges (startup, on connect) and, if you need reproducible gameplay randomness afterward, seed <a href="random-seed"><code>Random.seed</code></a> with it once.
+
+```ludic
+program Challenge {
+  entry {
+    let c = Crypto.random_u32()
+    Random.seed(value: c)          # non-deterministic seed, chosen once at startup
+    print(Random.int(100))
+  }
+}
+```