draft: stage remaining microflow roundtrip audit fixes

[hjotha]

Part of #332.

This is a draft staging PR for the remaining local microflow roundtrip validity work that has not yet been split into focused upstream PRs. It is not intended to be merged as-is.

Why this exists:

• keep the remaining local all-fixes branch visible upstream while focused PR extraction continues;
• provide a concrete branch for CI and maintainer discussion of the remaining control-flow, handler, split, type-inference, and validity fixes;
• avoid losing track of local work that is currently ahead of the already-open focused PR set.

Current status:

• build, unit tests, and Go lint pass locally on the staging branch;
• the current long-running audit uses an equivalent all-fixes binary as the best available local baseline;
• follow-up work should continue extracting focused PRs where possible.

Focused issues tracking the remaining staged work:

• Preserve enum split microflow roundtrips #347: enum split microflow roundtrips
• Preserve inheritance split and cast-action microflow roundtrips #348: inheritance split and cast-action microflow roundtrips
• Preserve custom error-handler routing during microflow roundtrip #349: custom error-handler routing during microflow roundtrip
• Preserve manual while-true retry-loop roundtrips #350: manual while-true retry-loop roundtrips
• Preserve nested branch tails and split-join continuations #351: nested branch tails and split-join continuations
• Preserve microflow type and cardinality inference during rebuild #352: microflow type and cardinality inference during rebuild

Validation run locally:

• make build
• make test
• make lint-go

Notes:

• This draft intentionally does not close Microflow roundtrip stabilization for large-project audit findings #332.
• Existing focused PRs should remain the preferred merge path where they cover the same behavior.
• The remaining issues above should be extracted from this staging branch into smaller PRs when their dependencies are isolated.