Add contributor-only Shuttle lock tests

[l1x]

Summary

This adds contributor-only Shuttle model-checking for DashMap's internal lock without changing the default runtime path.

What changed

• keep the existing parking_lot_core backend as the default build/runtime behavior
• add a shuttle-gated parking shim in src/lock.rs for model checking the lock under Shuttle
• model the Shuttle suite as an explicit [[test]] target with required-features = ["shuttle"]
• document Shuttle as a contributor-only workflow in the README instead of a normal user-facing feature
• add a dedicated CI job for cargo test --features shuttle --test shuttle
• add Shuttle coverage for reader/writer contention and downgrade() wakeup behavior

Tradeoffs

• shuttle still exists as a Cargo feature because the alternate lock backend needs conditional compilation in library code, but the intended surface is contributor-only rather than a supported end-user mode
• full cargo test --features shuttle remains intentionally unsupported; only cargo test --features shuttle --test shuttle is documented and exercised in CI
• Shuttle runs on stable in its own CI job so the pinned default toolchain and default-path checks stay unchanged

Verification

• cargo fmt --all
• cargo clippy --all-targets --features all -- -D warnings
• cargo test
• RUSTUP_TOOLCHAIN=stable cargo test --features shuttle --test shuttle

[eggyal]

I wonder if it wouldn't make more sense for this to be a wrapper within the shuttle project?

Oh, I see someone has submitted exactly that: awslabs/shuttle#279

[l1x]

@eggyal should we cancel this PR then?

[sarsko]

awslabs/shuttle#279 and this PR does different things.

This PR allows DashMap itself to be tested with Shuttle. awslabs/shuttle#279 is meant for users of DashMap to be able to run their application with Shuttle by just swapping their import in their Cargo.toml.

Regarding this PR:
There is a parking_lot wrapper in Shuttle that you could use:
https://github.com/awslabs/shuttle/tree/main/wrappers/parking_lot
If there is anything needed for DashMap that we currently don't have then please put in a PR and I'll review it.

[l1x]

@sarsko i was just wondering how could DashMap be tested with shuttle. I use Shuttle for my own code and wanted to expand to DashMap and I found that thread talking about it.