feat(stellar): add differential test harness for stealth-registry vs reference impl

[Tenny150]

Summary

Closes #110

Adds a property-based differential test comparing the on-chain stealth-registry Soroban contract against a pure-Rust reference implementation.

Changes

New files

stellar/stealth-registry/tests/reference/mod.rs — Pure-Rust reference implementation

• Models all contract operations: Register, Remove, Lookup, AdvanceLedger
• Faithfully models Soroban TTL semantics (extend threshold, expiry)
• Tracks application-level events (Register, Remove)
• Includes 8 comprehensive unit tests
• Documented as normative specification for expected on-chain behavior

stellar/stealth-registry/tests/differential.rs — Proptest differential test

• Generates random sequences of Register/Remove/Lookup operations (weighted 5:2:4)
• Executes each operation against both on-chain contract AND reference impl
• Asserts identical per-operation results including return values and error codes
• Asserts identical final state across all touched (registrant, scheme_id) pairs
• Respects WRAITH_PROPTEST_CASES env var (default 1024, nightly 16384)

Modified files

• .gitignore: Anchored reference/ pattern to root only, added patterns for test_snapshots/ and proptest-regressions/

Design Decisions

• AdvanceLedger excluded from proptest strategies due to Soroban SDK TTL invariants in the test environment. TTL expiration is modeled in the reference unit tests.
• Event comparison deferred to existing properties.rs tests. The reference tracks events internally for documentation and future use.
• Snapshot suppression via EnvTestConfig { capture_snapshot_at_drop: false } matching the existing properties.rs pattern.

Test Results

cargo test -p stealth-registry
# 34 passed, 0 failed, 3 ignored
# (audit: 4, differential: 11, properties: 6, registry: 6, upgrade_auth: 7)

Acceptance (from #110)

• Reference impl under stealth-registry/tests/reference/
• proptest generator emits sequences of ops; runs both, asserts identical final state
• Cover: register, revoke, expire, re-register
• cargo test --test differential passes with 1024 cases
• Reference impl documented as normative for expected semantics
• Failures print a minimized reproduction (proptest built-in)

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