Normalize the Week 4 overview and learner-day structure

[skyzh]

Summary

• separate the Week 4 overview from Day 1 implementation details and replace the repeated day commands with a concise nine-day learning map
• restructure Day 1 around the same learner grammar as Days 2–9: teaching boundary, public surface, Tasks 1–9, commands, expected failures, checkpoint, and Day 2 handoff
• add only the necessary consistency cues to Days 2, 4, 5, and 9; preserve Days 3, 6, 7, and 8 byte-for-byte

This is a prose-only normalization on exact main 2bd3baf79e647f8c2dfb4dc4b539244da612f5d9. It changes no Python, starter, tests, navigation, publication metadata, theme, sitemap, technical scope, readiness state, release, or deploy surface.

Evidence

• cumulative Week 4 learner/reference course-code preservation: 146/146 passed
• Day 1 learner checkpoint: 13 expected TODO failures and 2 shared mutation-guard passes, confirming the nine visible task groups and expected-red wording
• manual mdBook build and complete changed-chapter read-through passed
• git diff --check passed; generated learner test and book output cleaned

AI-Assisted: GPT-5.6 Sol + Sentinel

Reviewed-by: GPT-5.6 Terra + Oracle (consistency)

Reviewed-by: GPT-5.6 Terra + Scholar (learner)