Preserve MSRV-compatible atomic router metrics on current Rust

[Dastari]

Current Rust deprecates AtomicUsize::fetch_update, so warnings-denied router Clippy blocks the source-only workspace release (reproduced in #104 CI router job 110580382026). Replace that private saturating decrement with the equivalent relaxed compare-exchange retry loop. Avoid requiring the replacement try_update API, preserving the declared Rust 1.90 MSRV. Existing tests cover returning gauges to baseline and saturating at zero.

Advance router to 0.5.4 rather than reuse its already-published 0.5.3 package identity. No public API, protocol, configuration, stored data, dependency revision, license policy or binary distribution change. This is a separate compiler compatibility fix; it does not expand the ORM capability PRs.

#104 is merged and this PR targets main. Source-only workspace .3 will include the exact reviewed merge of both follow-ups. .2 remains immutable. No tag or release has been published for .3.

Validation:

• cargo test --locked -p graphql-orm-router --lib metrics::tests: two passed, 64 unrelated tests filtered out, none ignored.
• cargo clippy --locked -p graphql-orm-router --all-targets -- -D warnings: passed on current Rust.
• RUSTDOCFLAGS="-D warnings" cargo doc --locked -p graphql-orm-router --no-deps: passed.
• cargo +1.90.0 check --locked -p graphql-orm-router --lib: passed, complete library and dependencies.
• Documentation (191 files), generated inventory, formatting and package release policy pass.
• The router MSRV job on Repair ORM release fixture locks and execute PostgreSQL count authorization #104 passed; its current-Rust Clippy job reproduced the deprecation error. All eleven owning/other successful CI checks pass (including SMB, router default/auth tests, current-Rust Clippy/Rustdoc and MSRV); the companion check fails only on the independently tracked async-trait/Rust 1.99 issue Track Rust 1.99 Clippy compatibility for async-trait generated must-use attributes #107. Bind CI and release validation to reviewed Rust 1.97.1 #106 retains all gates on the reviewed Rust 1.97.1 baseline and must complete before publication.

Documentation impact

• Documentation updated
• No documentation impact

Explanation: patch changelog, no-action migration notes, aligned README version and generated inventory.