docs: add stress load validation plan

[div0rce]

Summary

Adds a stress/load validation plan for controlled degraded-condition firmware testing. This change is software-only and does not add new hardware evidence.

Changes

• Added docs/stress_load_validation.md with scenarios for UART-heavy telemetry, button bursts, ADC threshold flapping, telemetry delay, queue saturation, and cooperative-loop blocking.
• Defined stress metrics for missed deadlines, watchdog resets, queue pressure, jitter, loop time, heap, and stack margin.
• Kept the scope bounded as a validation plan, not measured timing closure or safety analysis.

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