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-v7.md

@@ -0,0 +1,23 @@
+---
+id: uuid-v7
+name: Uuid.v7
+category: uuid
+kind: namespace-method
+tokens: Uuid.v7
+sig: Uuid.v7() -> string
+tip: Explicit alias for Uuid.new_v7 — a time-ordered UUID.
+order: 4
+ns: Uuid
+member: v7
+---
+
+An explicit alias for <a href="uuid-new_v7"><code>Uuid.new_v7</code></a>: generates a **v7** (time-ordered) UUID whose leading bits are a millisecond timestamp so the IDs sort by creation time. Use this spelling to sit symmetrically beside <a href="uuid-v4"><code>Uuid.v4</code></a>. Behaviour, timestamp resolution, and the determinism caveat are identical to <code>Uuid.new_v7</code>.
+
+```ludic
+program V7 {
+  entry {
+    let id = Uuid.v7()
+    if Uuid.is_valid(id) { print(1) }
+  }
+}
+```