docs: define crypto hashing contract

[vycdev2]

Summary

• define the stable UTF-8-to-lowercase-hex SHA-256 and legacy MD5 contract
• classify planned SHA-512 and key-first binary HMAC, including secret taint, password non-claims, and compatibility behavior
• record the source/runtime boundary, backend obligations, implementation slices, and align design, architecture, progress, and docs indexes

Verification

• cargo test -q -p jett_driver run_pass_crypto — passed: the crypto driver fixture passed
• cargo fmt --check — passed
• cargo build — passed
• cargo test -q — passed: full workspace suite, including all 432 driver fixtures
• git diff --check — passed

Risk

• Medium: documentation-only, but it selects public crypto semantics and future API shapes that should be reviewed before implementation.

Closes #69

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[vycdev]

The crypto hashing contract reviewed as internally sound, but this documentation branch now conflicts with current main (including architecture text landed with #56). Please rebase and reconcile the current canonical text rather than overwriting it, then rerun diff/link checks.