[codex] Add engine_exchangeCapabilities support

[Kemperino]

Summary

• add engine_exchangeCapabilities to rollup-boost Engine API
• proxy capability negotiation to the local L2 execution engine
• add focused server test coverage for capability negotiation through rollup-boost

Validation

• cargo fmt --check
• git diff --check
• cargo test -p rollup-boost server::tests::engine_exchange_capabilities_success -- --exact
• cargo test -p rollup-boost server::tests
• cargo test -p rollup-boost test_integration_simple_isthmus
• docker build --platform linux/amd64 --build-arg RELEASE=true -t rollup-boost:exchange-capabilities-ea7fe88 .

[vercel]

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
rollup-boost	Ignored	Preview	May 23, 2026 1:13pm