docs: add technical defense notes

[div0rce]

Summary

Adds technical defense notes for explaining the actual firmware paths under review. This change is software-only and does not add new hardware evidence.

Changes

• Documented the ISR-to-task path, RTOS backend, cooperative backend, HAL boundary, fault state machine, watchdog strategy, queue behavior, and cooperative blocking risks.
• Added the required question bank with short answers, source files to inspect, failure modes, and better next steps.
• Kept the notes tied to current source paths instead of adding broad claims.

Validation Tasks

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