Docs: Clarify UTF-8 (no BOM) writer encoding policy and interoperability

[dborgards]

Background

Writer implementations now persist text using UTF-8 without BOM.

Problem

Project docs and historical assumptions still mention/assume DS 306 ASCII behavior in places.
Users need clear guidance on encoding defaults and interoperability expectations.

Proposed change

Document the library's encoding policy (UTF-8 no BOM) and explicitly explain interoperability tradeoffs for strict ASCII toolchains.

Acceptance criteria

• README/API docs clearly state default output encoding for all writers
• Compatibility guidance is provided for tools expecting strict ASCII
• Any intentional deviation from historical behavior is documented

[dborgards]

Closing manually after merge of PR #187 into develop. All CI checks completed successfully and the writer encoding policy/interoperability docs are merged.