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>

selfhost/emit_expr.ludic

@@ -235,6 +235,10 @@ function emit_ns_call(ns: pointer, meth: pointer, e: Node) -> Val {
     if is_crypto_ns(meth) { return emit_crypto_ns(meth, e) }
     perr(`unknown builtin Crypto.{meth}`)
   }
+  if (ns == "Uuid") {
+    if is_uuid_ns(meth) { return emit_uuid_ns(meth, e) }
+    perr(`unknown builtin Uuid.{meth}`)
+  }
   if (ns == "Vector") {
     if is_vector_ns(meth) { return emit_vector_ns(meth, e) }
     perr(`unknown builtin Vector.{meth}`)