docs: add legacy unity test guide

docs/TESTS.md

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+# Legacy Unity Tests
+
+This change intentionally keeps the existing PlatformIO legacy Unity harness unchanged.
+No `platformio.ini`, CI, or test-runner configuration was modified.
+
+## Compile-Only (Legacy Gate)
+
+Use compile-only checks in environments that do not have a connected board:
+
+```powershell
+pio test -e lilygo-t3-v1-6-1-test --without-uploading --without-testing
+pio test -e lilygo-t3-v1-6-1-868-test --without-uploading --without-testing
+```
+
+Suite-specific compile checks:
+
+```powershell
+pio test -e lilygo-t3-v1-6-1-test --without-uploading --without-testing -f test_html_escape
+pio test -e lilygo-t3-v1-6-1-test --without-uploading --without-testing -f test_payload_codec
+pio test -e lilygo-t3-v1-6-1-test --without-uploading --without-testing -f test_lora_transport
+pio test -e lilygo-t3-v1-6-1-test --without-uploading --without-testing -f test_json_codec
+pio test -e lilygo-t3-v1-6-1-test --without-uploading --without-testing -f test_refactor_smoke
+```
+
+## Full On-Device Unity Run
+
+When hardware is connected, run full legacy Unity tests:
+
+```powershell
+pio test -e lilygo-t3-v1-6-1-test
+pio test -e lilygo-t3-v1-6-1-868-test
+```
+
+## Suite Coverage
+
+- `test_html_escape`: `html_escape`, `url_encode_component`, and `sanitize_device_id` edge/adversarial coverage.
+- `test_payload_codec`: payload schema v3 roundtrip/reject paths and golden vectors.
+- `test_lora_transport`: CRC16, frame encode/decode integrity, and chunk reassembly behavior.
+- `test_json_codec`: state JSON key stability and Home Assistant discovery payload manufacturer/key stability.
+- `test_refactor_smoke`: baseline include/type smoke and manufacturer constant guard.
+
+## Manufacturer Drift Guard
+
+Run the static guard script to enforce Home Assistant manufacturer wiring:
+
+```powershell
+powershell -ExecutionPolicy Bypass -File test/check_ha_manufacturer.ps1
+```