docs(examples): add crypto hashing guest

[lntutor]

Summary

• add a standalone examples/crypto_hashing guest using random_bytes, hash_sha256, and hash_blake3
• encode values with a tiny dependency-free lowercase hex helper
• document build/run steps, nondeterministic random output, and the 32-byte digest sizes

Verification

• /Users/loi/.cargo/bin/cargo fmt --manifest-path examples/crypto_hashing/Cargo.toml --all --check
• /Users/loi/.cargo/bin/cargo build --manifest-path examples/crypto_hashing/Cargo.toml --release --target wasm32-unknown-unknown
• /Users/loi/.cargo/bin/cargo clippy --manifest-path examples/crypto_hashing/Cargo.toml --target wasm32-unknown-unknown -- -D warnings
• target/debug/nx run examples/crypto_hashing/target/wasm32-unknown-unknown/release/crypto_hashing.wasm
• /Users/loi/.cargo/bin/cargo fmt --all --check
• /Users/loi/.cargo/bin/cargo test --workspace

Closes #69

[lntutor]

Current head 321bc34ef80d2aa8ec3fbd6f236b700cd8e02382 is mergeable and the visible CI surface on this exact SHA is fully green: cross-platform Check, Clippy, and Test jobs on Ubuntu, macOS, and Windows; Format; Build WASM Examples; CLI Multi-process Smoke; Benchmark Comparator Tests; Ubuntu Profiling Artifacts; Benchmark Regression Gate; and the aggregate CI Success check all completed successfully. There are no review threads or review-body comments open on the current PR state, so if this crypto hashing guest example still matches the intended direction, it looks ready for maintainer review/merge when convenient.