refactor(bayn): make optional APIs pipeable

[gregkonush]

Summary

• Makes optional domain and boundary APIs pipeable while retaining their absence semantics.
• Keeps this native stack layer independently reviewable at 895 changed lines.
• Bases directly on codex/bayn-effect-tsgo-pipeable-schema so GitHub shows only this layer.

Related Issues

None

Testing

• bunx tsc --noEmit --project services/bayn/tsconfig.json
• bun run lint
• bun run lint:oxlint
• bun run lint:oxlint:type
• bun run build
• Focused tests for the touched subsystem were run while constructing the layer.

Breaking Changes

None

Checklist

• Testing section documents the exact validation performed.
• Screenshots are not applicable and Breaking Changes is filled in.
• Documentation and follow-ups are unchanged or represented by later stack layers.