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/_section.md

@@ -0,0 +1,13 @@
+---
+id: uuid
+title: Uuid
+order: 6
+---
+
+Universally-unique identifiers — stable IDs that don't collide, generated locally with no central authority handing out numbers. Reach for a UUID whenever something needs an identity that survives being saved, shared, or sent over a network: players and sessions in multiplayer, user-created content (levels, items, mods) that has to merge cleanly across installs, or a per-install / per-run ID for analytics and bug reports.
+
+Two versions are provided. <a href="uuid-new"><code>Uuid.new</code></a> makes a **v4** (random) UUID — 122 bits of entropy, effectively never colliding. <a href="uuid-new_v7"><code>Uuid.new_v7</code></a> makes a **v7** (time-ordered) UUID whose leading bits are a millisecond timestamp, so a batch of v7 IDs sorts by creation time — friendly to database indexes and append logs. Both set the RFC 4122 version and variant bits correctly.
+
+A UUID is represented as its canonical lowercase 36-character text form (<code>8-4-4-4-12</code>), the same shape you store, print, send, and compare — so there is no conversion at each boundary. Validate untrusted input with <a href="uuid-is_valid"><code>Uuid.is_valid</code></a> or normalise it with <a href="uuid-parse"><code>Uuid.parse</code></a>, and compare with <a href="uuid-equals"><code>Uuid.equals</code></a>, which ignores case.
+
+**Determinism caveat.** v4 and the random tail of v7 come from the OS cryptographically-secure RNG (<a href="ns-Crypto"><code>Crypto</code></a>), which is non-deterministic by design. Minting a UUID inside the lockstep simulation will desync replays and networked peers — generate IDs at the edges (on connect, on save, on spawn-from-input), never per tick in reproducible gameplay code.