docs: reduce README claim density

[div0rce]

Summary

Reduces README claim density and makes the landing page more engineering-native. This change is software-only and does not add new hardware evidence.

Changes

• Reorganized the README around overview, target, architecture, build/test flow, runtime behavior, timing evidence, layout, and limitations.
• Removed the long recruiter-style feature inventory from the README while keeping pointers to detailed evidence docs.
• Preserved bounded limitation wording around ESP-IDF, timing, certification, and hardware evidence.

Validation Tasks

• git status --short --branch
• git diff --check
• platformio test -e native
• scripts/check_app_core_includes.sh
• ESP-IDF RTOS backend build
• ESP-IDF cooperative backend build
• Claim-safety grep contains only explicit non-claim boundary wording