fix(chain): prevent overflow in KeychainTxOutIndex::lookahead_to_target

[CapThunder19]

Description

lookahead_to_target computed target_index + 1 with unchecked
addition. With target_index == u32::MAX, this panics in debug builds
and silently no-ops in release builds (wraps to 0, so the lookahead
request is quietly ignored).

Fixed by extracting the delta calculation into a lookahead_delta
helper using saturating_add instead of +, matching the pattern
already used elsewhere in this file.

Testing

Added lookahead_delta_does_not_overflow_at_u32_max, a unit test on the
helper directly. (Not an integration test calling lookahead_to_target
end-to-end with u32::MAX, since that would try to derive ~2^31 real
keys on a fresh index.)

• cargo test --features miniscript --lib
• cargo test --features miniscript --test test_keychain_txout_index
• cargo fmt --check -p bdk_chain
• cargo clippy -p bdk_chain --features miniscript --lib -- -D warnings

#2251