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/lib.rs

@@ -16,6 +16,8 @@
 
 pub mod codegen;
 pub mod config;
+#[cfg(feature = "crypto")]
+pub mod crypto;
 pub mod dictionary;
 pub mod error;
 pub mod ir;