Add scoped Stage 3 fit result bundles

[anth-volk]

Fixes #1045

Summary

• Add Stage 3 FittedWeightsInputBundle, FitResultBytes, and FittedWeightsOutputBundle types for scoped fit inputs/results.
• Keep _collect_outputs(...) as the legacy stdout-marker adapter while routing through the transport model.
• Convert regional and national remote results into scoped output bundles before writing files, and attach scope-specific fit diagnostics to step manifests.

Validation

• ruff check modal_app/pipeline.py modal_app/remote_calibration_runner.py policyengine_us_data/fit_weights/__init__.py policyengine_us_data/fit_weights/bundles.py tests/unit/fit_weights/test_bundles.py tests/unit/fit_weights/test_pipeline_docs.py tests/unit/test_pipeline_source_contracts.py
• ruff format --check modal_app/pipeline.py modal_app/remote_calibration_runner.py policyengine_us_data/fit_weights/__init__.py policyengine_us_data/fit_weights/bundles.py tests/unit/fit_weights/test_bundles.py tests/unit/fit_weights/test_pipeline_docs.py tests/unit/test_pipeline_source_contracts.py
• uv run --no-sync --with pytest --with pyyaml --with huggingface-hub python -m pytest tests/unit/fit_weights/test_bundles.py tests/unit/fit_weights/test_pipeline_docs.py tests/unit/test_pipeline.py tests/unit/test_pipeline_doc_guards.py tests/unit/test_pipeline_docs_extractor.py tests/unit/test_pipeline_source_contracts.py tests/unit/test_remote_calibration_runner.py
• uv run --no-sync --with pyyaml python scripts/run_quality_guards.py
• uv run --no-sync --with pyyaml python scripts/extract_pipeline_docs.py --json /private/tmp/stage3-pr3b-pipeline-docs/pipeline_map.json --api-json /private/tmp/stage3-pr3b-pipeline-docs/pipeline_api.json --markdown /private/tmp/stage3-pr3b-pipeline-docs/pipeline-map.md
• make lint
• uv run --no-sync --with towncrier towncrier check --compare-with agent/stage-3/pr-3a-specs-artifact-identity
• git diff --check