refactor(bayn): enforce exact TypeScript semantics

[gregkonush]

Summary

• enables the strict TypeScript compiler options that were still missing from Bayn, including exact optional properties, isolated modules, exhaustive returns, override checks, and side-effect import checks
• updates existing domain objects and test fixtures to preserve their exact runtime shapes under the stricter contracts
• keeps this as the first focused refactor layer above the Effect TSGo toolchain PR

Related Issues

None

Testing

• bun run --filter @proompteng/bayn tsc
• bun run --filter @proompteng/bayn lint
• bun run --filter @proompteng/bayn lint:oxlint:type
• bun run --filter @proompteng/bayn test
• bun run --filter @proompteng/bayn build

Breaking Changes

None

Checklist

• Testing section documents the exact validation performed (or N/A with justification).
• Screenshots and Breaking Changes sections are handled appropriately (removed or filled in).
• Documentation, release notes, and follow-ups are updated or tracked.