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-v4.md

@@ -0,0 +1,24 @@
+---
+id: uuid-v4
+name: Uuid.v4
+category: uuid
+kind: namespace-method
+tokens: Uuid.v4
+sig: Uuid.v4() -> string
+tip: Explicit alias for Uuid.new — a random (v4) UUID.
+order: 2
+ns: Uuid
+member: v4
+---
+
+An explicit alias for <a href="uuid-new"><code>Uuid.new</code></a>: generates a **v4** (random) UUID in canonical lowercase text form. Use this spelling when the surrounding code also uses <a href="uuid-v7"><code>Uuid.v7</code></a> and naming both by version reads more clearly than <code>new</code> versus <code>new_v7</code>. The behaviour, entropy, and non-determinism caveat are identical to <code>Uuid.new</code>.
+
+```ludic
+program V4 {
+  entry {
+    let a = Uuid.v4()
+    let b = Uuid.v4()
+    if a != b { print(1) }   # two draws differ
+  }
+}
+```