feat: Strengthen rt-ring testing with overwrite, concurrency, and loom checks

[hasanzakeri]

Summary

• Add stronger cross-thread tests in tests/concurrent.rs:
  • steady_state_no_overwrite_when_consumer_keeps_up proves no drops and zero overwrites when producer/consumer are in lockstep.
  • available_never_exceeds_capacity_under_pressure stress-tests a fast producer/slow consumer and asserts monotonic pops, available() <= capacity, and overwrites + popped == pushed.
• Tighten overwrite semantics and accounting in tests/overwrite.rs:
  • long_run_overwrite_accounting_and_tail_integrity pushes far beyond capacity and checks that only the last capacity items remain and overwrite_count is exact.
  • mixed_push_slice_pop_slice_matches_reference_and_overwrite_count interleaves push_slice/pop_slice with a VecDeque reference model and validates both ordering and overwrite counts.
• Extend property testing in tests/proptest.rs:
  tracks_available_and_overwrites_correctly keeps a reference queue in sync and asserts after every operation that available() and overwrite_count() match the model.
• Introduce optional loom model checks in tests/loom_spsc.rs:
  • Gated behind the loom-tests feature.
  • loom_small_interleavings_accounting_and_order and loom_capacity_one_keeps_latest_value explore small-state interleavings and assert monotonic order plus popped + overwrites == pushed.
• Wire loom into the build tooling:
  • Add a loom-tests feature in Cargo.toml.
    Add a test-loom target to Makefile that runs cargo test --features loom-tests --test loom_spsc via Docker.

Test plan

• make test
• make test-loom
• (CI) cargo miri test still exercises the existing tests, including the new property and concurrency cases.