Add composable trajectory token flags

[bradhilton]

Summary

• add public composable TokenFlag.EXACT and TokenFlag.SAMPLED facts
• replace TokenizedTrajectory.assistant_mask and sampled_spans with one aligned flags vector
• derive legacy training masks, sampled spans, MoE alignment offsets, and Tinker action masks at their consumption boundaries
• keep protocol-native CompletionsHistory.sampled_spans unchanged

Why

A token can carry independent provenance facts. An IntFlag preserves that composability without prescribing which positions downstream loss functions should train on. Consumers can select generated tokens with flags & TokenFlag.SAMPLED, while EXACT distinguishes IDs supplied by inference from client-side fallback tokenization.

Compatibility

This intentionally changes the new tokenized-trajectory interface. The known Caladan consumer is updated in a separate compatibility PR.

Validation

• 36 focused trajectory tests passed
• Ruff check and format check passed
• ty passed
• broader local unit collection is environment-limited by unavailable CUDA/Tinker optional dependencies; GitHub CI installs the repository's declared extras