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-is_valid.md

@@ -0,0 +1,28 @@
+---
+id: uuid-is_valid
+name: Uuid.is_valid
+category: uuid
+kind: namespace-method
+tokens: Uuid.is_valid
+sig: Uuid.is_valid(s) -> bool
+tip: Is s a well-formed UUID string?
+order: 6
+ns: Uuid
+member: is_valid
+---
+
+Reports whether <code>s</code> is a well-formed UUID: exactly 36 characters, hyphens at positions 8, 13, 18 and 23, and hexadecimal digits (either case) everywhere else. This is the guard to run on any UUID that came from outside your program — a save file, a network message, a mod, a command-line flag — before you trust it as an identity.
+
+It checks *shape*, not version: both v4 and v7 IDs (and any other conforming UUID) pass. To also fold a malformed value into a safe default in one step, use <a href="uuid-parse"><code>Uuid.parse</code></a> instead.
+
+Parameters:
+- `s` — the string to check
+
+```ludic
+program Valid {
+  entry {
+    if Uuid.is_valid("550e8400-e29b-41d4-a716-446655440000") { print(1) }
+    if not Uuid.is_valid("not-a-uuid") { print(2) }
+  }
+}
+```