docs: clarify supporting detection engineering role

[egrexsec]

Summary

• retain DetLab-DAC as an active detection-engineering workbench
• position it as the supporting lane for turning investigation findings and telemetry gaps into reviewable detections
• keep DFIR case management, evidence acquisition, storage, and forensic processing outside this repository
• correct documented Next.js and React major versions to match the current package manifest

Validation

• Node tests — 20 passed
• production Next.js build — passed
• git diff --check — passed

Boundaries

• no feature, detection content, schema, or runtime behavior changes