Add extensible hash primitives: SHA1, SHA256, SHA512

Introduces a `crypto` feature (on by default) that wires the RustCrypto
sha1/sha2 crates into a small `HashAlgo` registry. `register_primitives`
iterates `crypto::ALGOS` and installs one Forth host word per algorithm,
each with the stack effect

    ( c-addr u -- c-addr2 u2 )

reading `u` bytes from `c-addr` and writing the digest into a shared
`HASH_SCRATCH` region in linear memory (carved out between the float
stack and the dictionary).

Adding a new hash is a one-line entry in `ALGOS`. `register_host_primitive`
is now `pub` so downstream crates can extend the VM with their own I/O
host words without forking WAFER — kelvar (a deterministic password
manager on WAFER) is the first consumer.

- 4 unit tests (lib-level sha1/256/512 + registry sanity)
- 5 integration tests (in-VM `SHA1`/`SHA256`/`SHA512` against RFC-3174,
  FIPS-180, and the first-round S/KEY seed used by `hel`)
- All 437 existing lib tests still pass; `wafer-web` still builds for
  `wasm32-unknown-unknown` with the feature enabled

crates/core/src/crypto.rs

@@ -0,0 +1,111 @@
+//! Cryptographic hash primitives for WAFER.
+//!
+//! Provides a small registry of hash algorithms ([`ALGOS`]) and a Forth
+//! word per algorithm. Each word has stack effect:
+//!
+//! ```text
+//! ( c-addr u -- c-addr2 u2 )
+//! ```
+//!
+//! It hashes the `u` bytes starting at `c-addr` and writes the digest into
+//! the [`crate::memory::HASH_SCRATCH_BASE`] region. The output buffer is
+//! shared and overwritten by every subsequent hash call — copy the bytes
+//! out before the next invocation if you need to keep them.
+//!
+//! The registry is designed to grow: add a new entry to [`ALGOS`] and the
+//! word is registered automatically by
+//! [`crate::outer::ForthVM::register_crypto_words`].
+
+use sha1::{Digest as Sha1Digest, Sha1};
+use sha2::{Sha256, Sha512};
+
+/// One hash algorithm registered as a Forth word.
+pub struct HashAlgo {
+    /// Forth word name (uppercase by convention).
+    pub name: &'static str,
+    /// Digest length in bytes.
+    pub digest_len: usize,
+    /// Hash function.
+    pub hash: fn(&[u8]) -> Vec<u8>,
+}
+
+fn sha1_hash(input: &[u8]) -> Vec<u8> {
+    let mut h = Sha1::new();
+    h.update(input);
+    h.finalize().to_vec()
+}
+
+fn sha256_hash(input: &[u8]) -> Vec<u8> {
+    let mut h = Sha256::new();
+    h.update(input);
+    h.finalize().to_vec()
+}
+
+fn sha512_hash(input: &[u8]) -> Vec<u8> {
+    let mut h = Sha512::new();
+    h.update(input);
+    h.finalize().to_vec()
+}
+
+/// All hash algorithms registered as Forth words.
+pub const ALGOS: &[HashAlgo] = &[
+    HashAlgo {
+        name: "SHA1",
+        digest_len: 20,
+        hash: sha1_hash,
+    },
+    HashAlgo {
+        name: "SHA256",
+        digest_len: 32,
+        hash: sha256_hash,
+    },
+    HashAlgo {
+        name: "SHA512",
+        digest_len: 64,
+        hash: sha512_hash,
+    },
+];
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn hex(bytes: &[u8]) -> String {
+        let mut s = String::with_capacity(bytes.len() * 2);
+        for b in bytes {
+            s.push_str(&format!("{b:02x}"));
+        }
+        s
+    }
+
+    #[test]
+    fn sha1_rfc3174_abc() {
+        assert_eq!(hex(&sha1_hash(b"abc")), "a9993e364706816aba3e25717850c26c9cd0d89d");
+    }
+
+    #[test]
+    fn sha256_fips180_abc() {
+        assert_eq!(
+            hex(&sha256_hash(b"abc")),
+            "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad"
+        );
+    }
+
+    #[test]
+    fn sha512_fips180_abc() {
+        assert_eq!(
+            hex(&sha512_hash(b"abc")),
+            concat!(
+                "ddaf35a193617abacc417349ae20413112e6fa4e89a97ea20a9eeee64b55d39a",
+                "2192992a274fc1a836ba3c23a3feebbd454d4423643ce80e2a9ac94fa54ca49f"
+            )
+        );
+    }
+
+    #[test]
+    fn registry_lengths_match() {
+        assert_eq!(ALGOS[0].digest_len, sha1_hash(b"").len());
+        assert_eq!(ALGOS[1].digest_len, sha256_hash(b"").len());
+        assert_eq!(ALGOS[2].digest_len, sha512_hash(b"").len());
+    }
+}