refactor(firmware): colocate unit tests with implementations

[nicksan222]

Stack

Depends on #54. Review this PR against feat/firmware-linux-gpio-e2e (not main); only the test-location changes are included here.

Summary

• Colocate the pins, button subscription, event channel, hardware event, product-menu, SSID/passphrase, and harness unit tests with their implementations under apps/firmware/src/.
• Keep only cross-component E2E/Linux and display integration tests under apps/firmware/tests/.
• Preserve the test assertions and production behavior; update the firmware README to describe the distinction.

Verification

• just --justfile apps/firmware/justfile check (includes the Linux VM E2E suite)
• just --justfile apps/firmware/justfile yocto-check
• just --justfile apps/firmware/justfile cross-build

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-24T17:26:03.647229Z	a9d95f1	PR opened

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