Add shielded commitment and nullifier primitives

[TommoHCIO]

Summary

Addresses #33 by adding shielded commitment and nullifier storage primitives to the Soroban contract scaffold.

Changes

• Added persistent Commitment(BytesN<32>) and Nullifier(BytesN<32>) storage keys.
• Added insert_commitment with duplicate-commitment rejection and a typed CommitmentInserted event.
• Added consume_nullifier with replay prevention and a typed NullifierConsumed event.
• Added read helpers for commitment membership and nullifier-use checks.
• Documented the privacy-scaffold boundary in veilend-soroban/README.md.
• Added focused host tests for membership marking, duplicate commitment rejection, nullifier consumption, and nullifier replay rejection.

Verification

cargo fmt
cargo test -p veillend-contract
# test result: ok. 10 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

git diff --check
# no output

Notes

This intentionally does not implement zero-knowledge proof verification or token-note lifecycle logic yet. It establishes the stable storage keys and replay/duplicate guards that future shielded flows can build on.