docs: track crypto hashing contract

[vycdev2]

Summary

• link the Phase K crypto progress row to design issue design: define crypto hashing and security contract #69
• add backlinks from the hashing API and stdlib-boundary sections in the design and architecture docs
• preserve the existing design text while making the unresolved API, security, and runtime-boundary work explicit

Verification

• cargo fmt --check — passed
• cargo test -q -p jett_driver run_pass_crypto — passed (1 matching fixture test)
• cargo test -q -p jett_comptime test_sha256_known_vectors — passed
• cargo test -q -p jett_comptime test_md5_known_vectors — passed
• cargo build — passed
• cargo test -q — passed, including all 428 driver fixture tests
• git diff --check — passed

Risk

• Low: documentation-only backlinks; no language or runtime behavior changes.

Tracks #69

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.

[vycdev]

The documentation change looks good, but this branch now conflicts with main after #68, #72, and #74 updated adjacent standard-library tracker text. Please rebase and preserve the random, encoding, and UUID tracking links alongside this crypto contract link.