Consensus: reject duplicate Spark mint coins

[reubenyap]

PR intention

Prevent duplicate Spark mint coins from corrupting the relationship between Spark coin-group counts, block-index mint entries, and the active minted-coin map. A duplicate can otherwise be counted and indexed more than once while the map stores it once, making a later disconnect capable of removing the wrong occurrence or hitting an assertion.

Provenance: this PR splits and reconstructs the duplicate-mint hardening from public commit 28251a690, authored June 19, 2026 on the public spark-coin-type-fix branch. That work predates the later external vulnerability report. This PR carries Firo's earlier hardening forward as a separate, current-master change. It was not initiated in response to that report.

The earlier commit is not cherry-picked unchanged because it depends on its parent coin-type commit, only covers direct OP_SPARKMINT outputs within one block, and has no dedicated tests. This reconstruction is independent of #1901 and also covers OP_SPARKSMINT, active-chain duplicates, and mempool behavior.

Code changes brief

• Reject repeated Spark coin hashes across all mint-producing transactions in one block before any Spark state or block-index mutation.
• Reject a candidate active-tip block that repeats a Spark coin already present in active-chain state.
• Cover both direct OP_SPARKMINT coins and OP_SPARKSMINT coins created by Spark spends through the shared completed mint vector.
• Avoid false duplicate results during VerifyDB historical reconnection, where the UTXO view is rewound but global Spark state is not.
• Track Spark mint reservations and their transaction IDs independently in the main mempool and stem pool.
• Reject duplicate coins within one transaction or across mempool transactions without assigning peer misbehavior points.
• Remove a conflicting mempool transaction if an externally mined block confirms the same Spark coin.
• Gate block-consensus enforcement behind a separate activation height. Public-network and regtest defaults remain disabled at INT_MAX in this draft pending historical-chain review and coordinated deployment heights.
• Add tests for the activation boundary, both Spark coin types, active-chain and VerifyDB-style behavior, rejection before mutation, per-pool ownership, removal symmetry, and mempool duplicate policy.

Before this draft is made ready, the active histories must be scanned for pre-existing duplicate Spark coin identities and concrete network activation heights must be selected. git diff --check passes. Local CMake configuration reached dependency discovery but could not complete because the available Windows environment lacks bls-dash; compilation and the new tests therefore still require CI.

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