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>

examples/library/crypto.ludic

@@ -0,0 +1,30 @@
+# crypto.ludic — Crypto.* known-answer vectors + the secure-random surface.
+# SHA-256 / HMAC-SHA256 / base64 are checked against published test vectors, so a
+# regression in the integer IR shows up as a changed line. The secure-random
+# helpers are non-deterministic, so we assert their SHAPE (length, distinctness),
+# never a fixed value. Running it prints: 1 2 3 4 5 6 7 8 9
+program Crypto {
+  entry {
+    # SHA-256 (FIPS 180-4 examples)
+    if Crypto.sha256("abc") == "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad" { print(1) }
+    if Crypto.sha256("") == "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855" { print(2) }
+
+    # HMAC-SHA256 (RFC 4231 test case 2: key "Jefe", data "what do ya want for nothing?")
+    if Crypto.hmac_sha256("Jefe", "what do ya want for nothing?") == "5bdcc146bf60754e6a042426089575c75a003f089d2739839dec58b964ec3843" { print(3) }
+
+    # constant-time verify: the right MAC verifies, a tampered one does not
+    let mac = Crypto.hmac_sha256("k", "payload")
+    if Crypto.verify_hmac("k", "payload", mac) { print(4) }
+    if not Crypto.verify_hmac("k", "payload", "00") { print(5) }
+
+    # base64 (RFC 4648 vectors), covering every padding remainder
+    if Crypto.base64("") == "" { print(6) }
+    if Crypto.base64("f") == "Zg==" { print(7) }
+    if Crypto.base64("foobar") == "Zm9vYmFy" { print(8) }
+
+    # secure random: two 16-byte draws are 32 hex chars each and (near-certainly) differ
+    let a = Crypto.random_bytes(16)
+    let b = Crypto.random_bytes(16)
+    if len(a) == 32 and a != b { print(9) }
+  }
+}

examples/library/uuid.ludic

@@ -0,0 +1,32 @@
+# uuid.ludic — Uuid.* format round-trip, version/variant bits, validation and
+# equality. v4/v7 values are random, so we assert their STRUCTURE (length, the
+# version nibble, the variant, that two draws differ), never a fixed value.
+# Running it prints: 1 2 3 4 5 6 7 8 9 10
+program Uuid {
+  entry {
+    let a = Uuid.new()
+    let b = Uuid.new()
+
+    # 36 chars, canonical hyphen positions, and two draws differ
+    if len(a) == 36 { print(1) }
+    if a[8..9] == "-" and a[13..14] == "-" and a[18..19] == "-" and a[23..24] == "-" { print(2) }
+    if a != b { print(3) }
+
+    # v4: version nibble is '4' (index 14), variant is one of 8/9/a/b (index 19)
+    if a[14..15] == "4" { print(4) }
+    let var4 = a[19..20]
+    if var4 == "8" or var4 == "9" or var4 == "a" or var4 == "b" { print(5) }
+
+    # v7: version nibble is '7'; still a well-formed UUID
+    let c = Uuid.new_v7()
+    if c[14..15] == "7" and Uuid.is_valid(c) { print(6) }
+
+    # validation of untrusted input
+    if Uuid.is_valid("550e8400-e29b-41d4-a716-446655440000") { print(7) }
+    if not Uuid.is_valid("not-a-uuid") { print(8) }
+
+    # parse normalises case; equals is case-insensitive; bad input -> nil
+    if Uuid.equals(Uuid.parse("550E8400-E29B-41D4-A716-446655440000"), "550e8400-e29b-41d4-a716-446655440000") { print(9) }
+    if Uuid.parse("garbage") == Uuid.nil() { print(10) }
+  }
+}