[codex] Fail loud on malformed VM legacy tags

[proboscis]

Summary

Fixes vm-legacy-tag-parsers-fail-loud.

The legacy VM tag fallback parser no longer converts malformed control objects into plausible values. Missing payload attributes now fail loudly with tag-specific messages:

• Resume / Transfer: missing value
• Delegate / Pass: missing effect
• ResumeThrow / TransferThrow: missing exception

Regression coverage now lives in tests/core/test_vm_legacy_tag_parser_fail_loud.py, so it is covered by the repository's tests/ gate. I also added doeff-traverse build metadata so the full root suite can import that workspace package after uv sync --reinstall.

Root Cause

The legacy parser helpers used getattr(...).ok().map(...).unwrap_or(...), so missing Python attributes silently became Value::Unit or Value::String("unknown exception"). The outer match also replaced useful extraction errors with generic messages.

The reported packages/doeff-vm/tests/test_pyvm.py tests/ failure reproduces in this checkout as stale package-test API/export expectations (state, run, RunResult, start_program, etc.), not as a fail-loud live-path regression. Current Python constructors in doeff/program.py and the Rust pyclasses require the relevant payload fields, so absence of those attributes is malformed for the parser paths changed here.

Validation

• uv sync --reinstall
• uv run python -m pytest tests/core/test_vm_legacy_tag_parser_fail_loud.py -q -> 6 passed
• uv run python -m pytest tests/ -q -> 880 passed, 90 skipped