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-base64.md

@@ -0,0 +1,28 @@
+---
+id: crypto-base64
+name: Crypto.base64
+category: crypto
+kind: namespace-method
+tokens: Crypto.base64
+sig: Crypto.base64(s) -> string
+tip: Standard base64 (RFC 4648) of a string's bytes.
+order: 9
+ns: Crypto
+member: base64
+---
+
+Encodes the bytes of <code>s</code> as standard base64 (RFC 4648, the <code>A–Z a–z 0–9 + /</code> alphabet with <code>=</code> padding). Base64 turns arbitrary bytes into printable ASCII, which is what you want when a digest, key, or binary blob has to travel through a text-only channel — a JSON field, a URL-safe token wrapper, a config file, a log line. It is an *encoding*, not encryption: it hides nothing and adds no integrity. Pair it with <a href="crypto-hmac_sha256"><code>Crypto.hmac_sha256</code></a> when the payload must also be tamper-evident.
+
+The output length is always a multiple of four; a one- or two-byte remainder in the input is padded with <code>=</code>.
+
+Parameters:
+- `s` — the string whose bytes are encoded
+
+```ludic
+program Encode {
+  entry {
+    print(Crypto.base64("foobar"))   # Zm9vYmFy
+    print(Crypto.base64("f"))        # Zg==
+  }
+}
+```