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_bytes.md

@@ -0,0 +1,28 @@
+---
+id: crypto-random_bytes
+name: Crypto.random_bytes
+category: crypto
+kind: namespace-method
+tokens: Crypto.random_bytes
+sig: Crypto.random_bytes(n) -> string
+tip: n bytes from the OS CSPRNG, as a 2n-character hex string.
+order: 6
+ns: Crypto
+member: random_bytes
+---
+
+Draws <code>n</code> bytes from the operating system's cryptographically-secure random number generator and returns them as a <code>2n</code>-character lowercase hex string. Use it for unguessable tokens, nonces, and session secrets — anything whose whole value is that an attacker cannot predict it. The result is hex rather than raw bytes for the same reason digests are: a <code>str</code> is null-terminated and raw random bytes can contain a zero byte, so hex is the form you can safely store and compare.
+
+This is deliberately **non-deterministic** — it must never seed the lockstep simulation RNG (<a href="ns-Random"><code>Random</code></a>). Two calls return different values. On a platform without an OS CSPRNG (for example a bare wasm target) the draw degrades to zeroes rather than faulting; treat a real CSPRNG there as a platform-layer responsibility.
+
+Parameters:
+- `n` — the number of secure random bytes to draw
+
+```ludic
+program Token {
+  entry {
+    let session = Crypto.random_bytes(16)   # 32 hex chars, unguessable
+    print(len(session))                      # 32
+  }
+}
+```