[08.6] Make vendor purchases fully atomic

[alseif0x]

Closes #108

What changed

• Plans vendor currency gains, currency costs, item turn-ins, inventory, and gold without publishing partial runtime state before durable success.
• Commits all purchase-owned CharacterDB mutations atomically and quarantines cancelled or indeterminate COMMIT outcomes for reload.
• Matches the C++ vendor item metadata, packet order, and realm/instance routing.
• Hardens cross-socket post-COMMIT fanout so transient write backpressure does not suppress already-committed responses.
• Adds focused tests, paired C++/Rust bot QA, retained capture provenance, and a strict clean vendor-purchase capture fixture.

Why

The currency-vendor path and item extended-cost path previously published currency changes before the database COMMIT completed. A rollback, cancellation, or lost COMMIT response could leave runtime and durable inventory/currency state inconsistent.

Validation

• ./tools/pr-preflight.sh full origin/3.4.3 passed with Rust 1.88.0 and pinned protoc.
• Full wow-world, network, map/entity, focused vendor, bot, and capture-diff suites passed.
• Local exhaustive Codex review: CLEAN.
• Paired real C++/Rust bot run proved currency 30 -> 15, one durable item after fresh authentication, correct packet routing, and complete fixture cleanup.
• Installed original-client QA passed two extended-cost purchases across relog; CharacterDB retained exactly one of each item and the correct currency balance, then the character fixture was fully restored.

Boundaries

Finite-stock oversell (D-H11), buyback/refund, the missing achievement SMSG_CRITERIA_UPDATE, and broader vendor validation remain separately scoped.

[alseif0x]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Hooray!

Reviewed commit: fc85e0c197

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