fix(rs-platform-wallet): reserve platform receive address on hand-out (Found-026) [backport]

[lklimek]

Semantic backport to v3.1-dev of the Found-026 reserve-on-hand-out fix already proven on #3549 (3/3 focused 14-thread + R2-clean + adjudicated).

Problem

key-wallet AddressPool::next_unused returns the first used==false index WITHOUT marking it; used only flips on a positive synced balance. Two concurrent callers of PlatformAddressWallet::next_unused_receive_address therefore receive the SAME address before the sync pass — duplicate-address race.

Fix

Reserve the handed-out index under the already-held wallet_manager.write() guard: swap next_unused(...) for next_unused_with_info(...), then call mark_index_used(info.index) before releasing the guard. mark_index_used is idempotent, so a later real sync hit on the index is a no-op (no gap-limit / highest_used double-counting).

Why hand-applied (not cherry-picked)

The next_unused_receive_address region diverged on v3.1-dev (different rust-dashcore pin, no Found-026 doc block, direct-Address return) so a textual cherry-pick of the #3549 commit does NOT apply. Same SEMANTIC transform re-applied by hand to v3.1-dev's own structure; verified the v3.1-dev key-wallet pin (53130869e5b...) exposes byte-identical AddressPool (next_unused_with_info, mark_index_used, AddressInfo) to the proven pin (5297d61...).

Validation

cargo check (lib + --tests), cargo clippy --lib -D warnings, cargo test --lib (121 passed, 0 failed), rustfmt — all clean. Behavioural correctness proven on #3549; v3.1-dev has no e2e harness / found_026 guards (those are #3549-only) so this PR confirms compile + no lib-test regression.

Backport sibling of #3549 / Found-026.

🤖 Generated with Claude Code

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