[codex] add guarded ReadersWriters reference app

[arun-pathiban-ddog]

Summary

Adds a Temper ReadersWriters reference app that faithfully mirrors the Quint transition relation through a small WASM transition module and guarded IOA callbacks.

What changed

• Added reference-apps/readers-writers with the IOA spec, CSDL model, Cedar policy, pure Rust transition logic, trace tests, validation tests, and WASM step module.
• Added a wasm_in_flight guard to serialize public protocol actions while a WASM callback chain is open, preventing stale overlapping proposals from drifting detailed queue fields from projected counters.
• Updated server-hosted Monty REPL loopback calls to preserve tenant and identity headers.
• Made inline WASM callback dispatch wait through nested callback triggers so public action flows can observe the committed callback state.

Impact

The reference app demonstrates ReadersWriters behavior under Temper verification: multiple readers can share the resource, writers wait for readers, readers wait for writers, and no reader/writer overlap is admitted. Concurrent public calls now get visible backpressure through the guard instead of racing detailed WASM-managed fields.

No HTML report files are included in this PR.

Validation

• cargo fmt --all --check
• cargo test -p readers-writers-reference
• cargo test -p temper-sandbox
• cargo build -p temper-cli