Simplify build workflow and apply saved modality corrections

[Cat-blizzard]

Summary

• Organize the build into five phases with option-aware progress and skip redundant teaching enrichment.
• Keep Streamlit progress responsive when the build prints no output.
• Carry saved page modality corrections to the next build of the same source, validate source hashes, and apply corrections before visual processing.
• Write the final element IR after coverage is known and update the pipeline documentation.

Verification

• .venv\Scripts\python.exe -m pytest -q — 262 passed.
• git diff --check — passed.