Re-enable bridge load tests with deterministic isolation

Problem Statement

tests/bridge_load_tests.rs:226 and tests/integration_bridge_oracle.rs have #[ignore] due to test isolation.

Why it matters

Load tests are the empirical basis for our gas-budget claims; if they're ignored, we have no load signal.

Technical Context

Fix the isolation (#39) then unignore; rewrite load assertions to be deterministic (use Instant::now of mocked env, not wall-clock).

Expected Outcome

Full suite runs in CI; ≥ 10k requests in < 10s documented in result.

Acceptance Criteria

1. cargo test --release load_test_concurrent_registration_light green.
2. cargo test --release stress_test_mass_registration green.
3. Output captured to docs/PERFORMANCE.md.

Implementation Notes

Use per-test Arc<Mutex<…>> harness with set_block_number(0) reset.

Files or modules likely to be affected

tests/bridge_load_tests.rs, tests/integration_bridge_oracle.rs.

Dependencies

#39.

Difficulty level

HARD.

Estimated effort

M (~3 days).