test: add C portable fault classifier module

[div0rce]

Summary

Adds a small deterministic pure-C fault classifier module with host-native Unity coverage. This change is software-only and does not add new hardware evidence.

Changes

• Added fault_classifier.h / fault_classifier.c under components/app_core.
• Added native Unity tests for invalid config, ADC low/high faults, stale samples, valid samples, and boundary behavior.
• Wired the module into ESP-IDF app-core CMake and PlatformIO native source filtering, and documented it in the evidence map.

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

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Classify timeout-boundary samples as stale

Use >= for the stale-age check so samples at exactly stale_timeout_ms are treated as stale; with the current > condition, sample_age_ms == stale_timeout_ms returns FAULT_CLASSIFIER_OK, which is inconsistent with the existing watchdog timeout semantics (elapsed_ms_u32 uses >= and backend fault checks trigger at the configured timeout). In scenarios where age lands exactly on the boundary, this delays stale-fault detection by one tick.

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