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-to_text.md

@@ -0,0 +1,26 @@
+---
+id: uuid-to_text
+name: Uuid.to_text
+category: uuid
+kind: namespace-method
+tokens: Uuid.to_text
+sig: Uuid.to_text(id) -> string
+tip: The canonical text form of a UUID.
+order: 7
+ns: Uuid
+member: to_text
+---
+
+Returns the canonical 36-character text form of <code>id</code>. Because this library already represents every UUID as its canonical lowercase text, <code>to_text</code> is the identity function — it exists so intent is explicit at the point where a UUID is turned into a string for display, storage, or a wire payload, and so code stays correct if the internal representation ever changes to a packed 128-bit value.
+
+Parameters:
+- `id` — the UUID to render
+
+```ludic
+program ToText {
+  entry {
+    let id = Uuid.new()
+    print(len(Uuid.to_text(id)))   # 36
+  }
+}
+```