fix: bound M0 publisher envelopes to ingress size

[Pigbibi]

Summary

• reject a canonical M0 publisher envelope above the Worker ingress 256KiB limit
• apply the same body representation to local file output and explicit publish
• fail before writing, reading publish variables, or sending a network request

Safety

• maintains offline-by-default behavior
• no Worker, UI, scheduler, runtime, strategy, platform, or secret configuration change

Validation

• python -m unittest tests.test_build_m0_research_publisher_envelope passed locally: 7 tests
• Python compile, schema parse, and diff checks passed
• GitHub CI will run full regression checks