docs: add plan for workflow chaining

[andreatgretel]

Summary

• Adds a design plan for workflow chaining - sequencing multiple generation stages where each stage's output seeds the next.
• Primary use cases: explode (few seeds -> many records), filter-then-enrich, generate-then-judge, multi-turn construction.
• Secondary benefit: enables removal of allow_resize and simplification of sync/async engine convergence.

What's in the plan

• Pipeline class in the interface layer with add_stage(), run(), between-stage callbacks
• to_config_builder() convenience on results for lightweight notebook chaining
• allow_resize removal (phased, after pipeline ships)
• Pre-batch processor resize lockdown (fail-fast on row-count changes)
• Stage-level checkpointing and resume with fingerprint-based invalidation
• Four concrete use cases with code examples
• Open questions on image forwarding, fan-out semantics, and downstream seeding scope

Phases

1. Pipeline class + to_config_builder() (can ship independently)
2. Remove allow_resize (deprecate first, then remove)
3. Stage-level resume
4. Auto-chaining from single config (future)

No code changes - plan document only.