Add Week 3 R setup and shared observations route

[mhasoba]

Implements the approved Phase 2 teaching route. Adds platform-neutral R installation and user-library guidance, replaces unsafe sudo-R and system-profile advice, adds an explicitly unassessed guided/independent/critique observations exercise, and runs a base-R fixture contract check in CI.

Validation:

• Local make check-content passed with zero issues.
• Local make test built the book successfully with 19 pre-existing warnings.
• GitHub Actions run 38080684937 passed R setup, the shared observations contract, content checks, the normal book build, and artifact upload.

At the user direction, strict forced execution of all historical notebooks is temporarily suspended; issue #205 records the evidence and revival checklist. The retained checks remain active. This PR still requires independent teaching-owner approval before merge.

[mhasoba]

Closing as premature. The R chapter and related MQB material require further local revision before a new review.