fix(key-wallet): refuse BIP-32 depth past 255 instead of overflowing #1298

ExtendedPrivKey::ckd_priv and ExtendedPubKey::ckd_pub compute the child depth as self.depth + 1 on a u8 (key-wallet/src/bip32.rs). The same pattern is in derivation_bls_bip32.rs and derivation_slip10.rs.

DerivationPath::from_str accepts paths with more than 255 components, so deriving m followed by 256 /0 steps panics in builds with overflow checks. Without overflow checks, the depth wraps to 0 and a serialized xpub carries the wrong depth.

Found in review of dashpay/platform#5727, whose helpers derive at caller-supplied path strings.

Proposed fix: use checked_add and return a typed depth-overflow error from child derivation (or reject paths deeper than 255 when parsing), and test depths 255 and 256.