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

[evanlinjin]

Thanks for the PR. Concept ACK.

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.40%. Comparing base (337e9d6) to head (3e76f04).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
crates/chain/src/indexer/keychain_txout.rs	92.85%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2253      +/-   ##
==========================================
+ Coverage   78.36%   78.40%   +0.03%     
==========================================
  Files          30       30              
  Lines        5945     5955      +10     
  Branches      281      281              
==========================================
+ Hits         4659     4669      +10     
  Misses       1210     1210              
  Partials       76       76              

Flag	Coverage Δ
rust	78.40% <92.85%> (+0.03%)	⬆️

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[CapThunder19]

pls review it

[evanlinjin]

#2265 supersedes this.