refactor(config): bound recipe composition

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Summary

• Replace copied full-tree YAML in 110 public recipes with one bounded layer of 19 family/workflow bases under examples/_base/; all 120 existing --config-name entry paths remain unchanged.
• Keep experiment-defining algorithm, engine, topology, model, and data overrides in each public entry. A public entry may select one base followed by _self_; bases cannot inherit.
• Add a pre-commit guard that validates this graph and asks Hydra to compose every public entry.
• Update contributor/config documentation for the bounded composition rule.

This reduces recipe YAML from 22,813 to 15,129 lines. Including the guard and documentation, the PR is 2,413 insertions and 9,942 deletions (net -7,529 lines).

This is stacked on refactor/diffusion-runner (#29 in this fork), not on main.

Related Issue

N/A

Test Plan

• pre-commit run --all-files — passed, including all architecture/framework/experimental guards, target resolution, YAML validation, and the new 120-entry composition guard.
• python -m compileall -q lint unirl — passed.
• python lint/check_recipe_targets.py — 1,154 recipe _target_ paths resolve.
• python lint/check_architecture_boundaries.py — 475 files / 4,273 internal import edges satisfy all eight boundaries.
• python lint/check_framework_contracts.py — passed.
• One-off Hydra 1.3.2 parity harness composed every public config from the parent commit and this commit and compared OmegaConf.to_container(..., resolve=False) recursively — 120/120 exact structural matches.
• Not run: GPU training or rollout smoke. This PR changes recipe source composition only; the composed configs are structurally identical.

Compatibility / Risk

• Supported public recipe paths, CLI overrides, _target_ values, checkpoints, datasets, and runtime math are unchanged.
• Tools that read a public YAML file directly with a generic YAML parser will now see only local overrides; they must compose it with Hydra to obtain the effective config. Hydra entrypoints and launchers already do this.
• examples/_base/ paths are private implementation details and are not supported experiment entrypoints.
• The graph is intentionally restricted to one base per public entry. This avoids deep inheritance and makes override order explicit.

Reviewer Notes

• Suggested review order: lint/check_recipe_composition.py, one representative base/entry pair such as SD3, then the documentation. The remaining YAML edits are the same mechanical extraction and are covered by 120/120 parity.
• Duplicate-work check: official fix(config): make the cross-component recipe contracts actually gate Tencent-Hunyuan/UniRL#257 changes runtime cross-component validation and overlaps only in config documentation/pre-commit surface; it does not introduce recipe composition. Official refactor(types): drop stage_config backward compatibility Tencent-Hunyuan/UniRL#263 is unrelated. No open PR was found that extracts shared recipe bases. Official issue [RFC] Refactor and simplify the video track Tencent-Hunyuan/UniRL#285 is the broader video refactor RFC.
• AI assistance was used for the mechanical extraction, guard, and parity harness. The complete diff and validation results were reviewed before publication.

Checklist

• I reviewed the changed code and removed unrelated/generated artifacts.
• I updated tests, docs, and configs where needed, or explained why not.