Add critical technical editor review (REVIEW-20260424b)

Comprehensive independent technical review covering:
- 15 cross-cutting issues (heading levels, dead links, missing index/exercises, etc.)
- Per-chapter issues with priority ratings
- Specific bugs: missing ');' in Java code (ch20), 'ccoverageExcludedPackages' typo (ch20)
- Image quality issues: SOS rules as slide screenshots (ch50), whiteboard photos
- Stale content: SE Radio feedproxy links (ch60), Java 8 API link, Scala 2 REPL transcripts
- Structural assessment with strengths and weaknesses
- Priority-ranked fix list (IMMEDIATE / HIGH / MEDIUM / LOW)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: klaeufer