feat(keys)!: add derivation for message-signing and fallback keys

[nchamo]

Why we are doing this

#23510 added the message-signing (mspk) and fallback (fbpk) keys to the protocol PublicKeys, but baked them into the address commitment as fixed default values with no derivation path. This gives both keys real per-account derivations.

The message-signing key is the prerequisite for interactive handshakes: a recipient's consent signature is verified against mspk.

Our fix

• deriveKeys derives mssk/fbsk and populates the mspk/fbpk hashes from them, instead of the shared default constants.
• The key store persists and exposes both keypairs; the now-repetitive getters are collapsed behind a single helper.

Because the mspk/fbpk hashes feed PublicKeys::hash(), every account address derived from a secret changes — hence the breaking flag. Standard and protocol contracts derive from PublicKeys::default() (unchanged), so their addresses are unaffected.