fix(pairing): enforce TXT record value formats

[kirollosnct]

Validate() only checked the enum fields (alg, parType, type, trustCurve). The hexadecimal values were accepted in any case and length: forPar was matched case-insensitively, trustPar was never validated at all, and trustNonce/digest only had to survive a lenient hex decode.

This matters because the shippairing digest is calculated over the TXT values as announced: a record whose values are malformed in a way that survives lenient parsing — lowercase hex, wrong-length nonce — carries a digest that still verifies, so format validation is the only gate that can reject it. TC_SPS_TXT_009 of the pairing test spec V1.0.0 tests exactly this (digest computed over already-malformed values) and its final release extends the trustNonce matrix with valid-hex wrong-length errors (E14/E15) that a hex decode cannot catch.

Enforce the formats of pairing spec section 5.4, Table 1 in Validate(): forPar, trustPar and digest must be 64 uppercase hexadecimal digits, trustNonce 32; forId and trustId must be non-empty; txtvers must be "1". Validate() runs at the mDNS discovery gate, in the listener and on our own announcements, so every evaluation path is covered by one change.

Test fixtures with placeholder values ("trust-1-par", short nonces) are updated to conforming formats; a new matrix test mirrors every row of TC_SPS_TXT_009 Table 5 with the error types of test spec section 2.8.9.

[coveralls]

coverage: 92.847% (-0.1%) from 92.972% — fix/pairing-txt-format-validation into dev