feat(test): add deterministic test harness and swap_invariant example…

[OTimileyin]

Summary

Closes #8

Adds a test-determinism feature gate and a reusable TestHarness that provides deterministic ledger timestamps, event tracing, and Jest-style timing artifacts in CI.

Changes

peerx-contracts/counter/Cargo.toml

Added test-determinism = [] feature flag.

peerx-contracts/counter/src/test_harness.rs (new)

TestHarness struct with:

• Deterministic env – pins ledger timestamp to 1_700_000_000 and sequence to 1 for bit-for-bit reproducibility
• set_timestamp(ts) – advance the ledger mid-test (same with_mut pattern from rate_limit.rs)
• with_event_trace(f) – captures all Soroban events emitted during f, so failing assertions can be tied to a specific event sequence
• generate_address() – convenience wrapper for Address::generate
• Timing artifacts – on Drop, writes $CARGO_TARGET_TMPDIR/timing/<name>.json (Jest-style)

peerx-contracts/counter/src/lib.rs

Registered mod test_harness behind #[cfg(all(test, feature = "test-determinism"))].

peerx-contracts/counter/src/invariants.rs

Added swap_invariant test demonstrating the harness: happy path, timestamp determinism, event tracing (no-op + mint), and invariant verification.

Acceptance Criteria

• Jest-style test artifacts (CARGO_TARGET_TMPDIR/timing) are published
• swap_invariant example test demonstrates the harness

Verification

• Cargo.toml passes TOML validation
• Only the four named files were touched

[ameeribro4-sudo]

LGTM