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-parse.md

@@ -0,0 +1,29 @@
+---
+id: uuid-parse
+name: Uuid.parse
+category: uuid
+kind: namespace-method
+tokens: Uuid.parse
+sig: Uuid.parse(s) -> string
+tip: Normalise an untrusted string to a lowercase UUID, or the nil UUID.
+order: 5
+ns: Uuid
+member: parse
+---
+
+Normalises an untrusted string: if <code>s</code> is a well-formed UUID it returns the same value in canonical lowercase form; if it is not, it returns the <a href="uuid-nil"><code>nil</code></a> UUID. This never faults on garbage, so it is safe to call on data that arrived from a file, a network peer, or a mod. When you need to *reject* bad input rather than fold it to nil, gate on <a href="uuid-is_valid"><code>Uuid.is_valid</code></a> first.
+
+Parsing accepts either case and yields the lowercase canonical form, which is what the rest of the library produces — so a parsed ID compares equal (with <code>==</code>) to a freshly generated one of the same value.
+
+Parameters:
+- `s` — the string to validate and normalise
+
+```ludic
+program Parse {
+  entry {
+    let id = Uuid.parse("550E8400-E29B-41D4-A716-446655440000")
+    print(id[0..8])                          # 550e8400 — lowercased
+    if Uuid.parse("nope") == Uuid.nil() { print(1) }
+  }
+}
+```