refactor(core): carry typed host errors

[yashranaway]

Closes #22

Summary

• return browser-operation failures from the isolated runtime in a bounded JSON envelope with an allowlisted code
• unwrap both WebKit and CDP results into one shared HostError with the protocol code and recovery suggestion
• remove host-side semantic message.contains(...) classification and fail unknown codes closed
• replace the recording startup test's flaky wall-clock assertion with an exact bounded-attempt assertion

Validation

• npx --yes pnpm@9.15.0 test:runtime
• Swift 6.3 protocol suite in Docker (42/42)
• npx --yes pnpm@9.15.0 test:e2e:linux
• git diff --check