docs(protocol): clarify OwnerPSK vector scope

[Jason-Morcos]

Summary

• rename the two fixed-output tests so they are clearly synthetic regression vectors rather than captures from IoTivity or a device
• record what those values do and do not prove about the derivation
• document that IoTivity's other 96-byte ECDH_ANON, ECDHE_PSK, and ECDHE_RSA mappings are intentionally outside this helper's manufacturer-certificate allowlist

Why

This follows up on the two non-blocking notes in the merge comment on #45. The implementation is unchanged; the test provenance and cipher scope are now explicit for the next reader.

Validation

• SmartThings-Local: 253 tests on Python 3.14
• dependency floor: 253 tests on Python 3.11 with the minimum declared versions
• current LocalThings main: 1,625 tests
• wheel/sdist install checks, distribution, compile, and share-safety checks

[QuiteYellow]

Both notes covered, thanks.

Merging.