Add Stage 2 saved-run artifact loaders and readiness reports

[anth-volk]

Fixes #142

Summary

Follow-up to the canonical US runtime stage taxonomy proposed in #20.

This adds the Stage 2 saved-run artifact/readiness layer:

• extends stage artifact contracts with path hints, formats, hash modes, and resume roles
• writes typed per-stage output manifests under stage_artifacts/manifests/
• writes stage_manifest.json, stage_artifacts/artifact_inventory.json, and stage_artifacts/conditional_readiness.json from saved artifact bundles
• adds stage artifact loaders, checked path resolution, hashing, and readiness classification helpers
• wires the saved-run artifacts into versioned artifact saving, PE-US-data checkpoint evidence, backfill paths, experiment summaries, data-flow snapshots, and docs/API exports
• tightens stage input/output validation, explicit override handling, Stage 8 dataset completeness, and Stage 9 evidence/summary handling
• adds contract/schema alignment tests so contract outputs/artifacts stay aligned with typed stage manifests

Out of scope

• live manifest updates during the canonical build
• start-of-stage runtime gates
• automatic conditional execution or scheduling
• partial resume inside calibration/synthesis
• statistical/modeling behavior changes

Validation

Local focused checks after rebasing on origin/main:

• ruff format --check src/microplex_us/pipelines/stage_run.py tests/pipelines/test_stage_run.py
• ruff check src/microplex_us/pipelines/stage_run.py tests/pipelines/test_stage_run.py
• /opt/miniconda3/bin/python -m py_compile src/microplex_us/pipelines/stage_run.py tests/pipelines/test_stage_run.py
• git diff --check

Local limitation: targeted pytest collection is blocked in this environment by ModuleNotFoundError: No module named 'microplex'; uv run pytest is also blocked by the macOS x86_64 torch==2.11.0 wheel resolution issue encountered earlier.