feat(stdlib): Crypto.* — SHA-256 + HMAC-SHA256, constant-time verify (#19)

The security-sensitive counterpart to the fast, non-cryptographic Hash.*
library: standard, test-vector-backed hashing for signed saves and message
integrity, kept in its own namespace so nobody reaches for the wrong tool.

  Crypto.sha256(s)                SHA-256 -> 64-char lowercase hex
  Crypto.hmac_sha256(key, msg)    HMAC-SHA256 -> 64-char hex
  Crypto.verify_hmac(key, msg, mac)  recompute + constant-time compare -> bool
  Crypto.hex(s)                   lowercase hex of a string's bytes
  Crypto.ct_equal(a, b)           constant-time string equality

The primitives are implemented from scratch in plain integer LLVM IR
(FIPS 180-4 / RFC 2104): no libc crypto, no data-dependent branches in the
compression rounds, so a given input hashes to the same 32 bytes on every
platform and run. Digests are returned as hex strings, not raw bytes, because
a `str` is null-terminated and a raw digest can contain a NUL. MAC checks use
a non-short-circuiting compare so timing does not leak how much of a forged tag
was correct.

Emitted on demand via g_uses_cryptort, mirroring the emit_hash prelude gate.
Scoped to the deterministic, known-answer-testable core; OS-backed
random_bytes (the one piece that can't be validated by test vectors) is left
for a follow-up.

Tested against published SHA-256 vectors (empty/"abc"/fox + 55/56/64-byte
multi-block padding) and HMAC-SHA256 vectors; wired into the self-host suite as
`crypto`. Docs: a new Crypto section with honest "what this protects / does
not" guidance, one page per method, all fences checked and in the inventory.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/language/crypto/crypto-ct_equal.md

@@ -0,0 +1,27 @@
+---
+id: crypto-ct_equal
+name: Crypto.ct_equal
+category: crypto
+kind: namespace-method
+tokens: Crypto.ct_equal
+sig: Crypto.ct_equal(a, b) -> bool
+tip: Constant-time string equality for secrets.
+order: 5
+ns: Crypto
+member: ct_equal
+---
+
+Compares two strings for equality without short-circuiting: every character is examined even once a difference is found, so the time taken does not reveal where — or whether — the strings first diverged. Reach for it whenever you compare a secret, token, or MAC that an attacker might be probing. For the common case of checking a message tag, <a href="crypto-verify_hmac"><code>Crypto.verify_hmac</code></a> already does this for you; use <code>ct_equal</code> directly when you hold both values yourself. Strings of different length return <code>false</code> at once (length is not secret). For ordinary, non-secret text just use <code>==</code> — the constant-time guarantee is not free.
+
+Parameters:
+- `a` — one string
+- `b` — the other string
+
+```ludic
+program CompareToken {
+  entry {
+    if Crypto.ct_equal("abc", "abc") { print(1) } else { print(0) }   # 1
+    if Crypto.ct_equal("abc", "abd") { print(1) } else { print(0) }   # 0
+  }
+}
+```