[UTXO-BUG] Support legacy balance schema during epoch reward settlement

[Zephyr-Echo]

Summary

finalize_epoch() updated balances using miner_id/amount_i64 unconditionally.
Legacy databases created by init_db() use miner_pk/balance_rtc, so enabling
UTXO_DUAL_WRITE caused epoch settlement to roll back with:

sqlite3.OperationalError: no such column: amount_i64

Fix

Reuse the existing schema-tolerant balance helpers and add a regression test for
legacy balances with UTXO dual-write enabled. The patch preserves the existing
no-phantom invariant: miners without an account row are not credited or ledgered.

Validation

• python -m pytest -q node/tests/test_integrated_balance_scale.py
• python -m pytest -q node/test_utxo_db.py node/tests/test_epoch_utxo_dual_write_guard.py
• python -m py_compile node/rustchain_v2_integrated_v2.2.1_rip200.py

wallet: RTC6340d6bd7fd0804ad8afb1f319f826acc784ed26

[github-actions]

Welcome to RustChain! Thanks for your first pull request.

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[Zephyr-Echo]

Thanks for reviewing. The change is limited to schema-compatible epoch reward
settlement and preserves the existing no-phantom balance invariant. No live
network testing was performed. I will keep payout details and private contact
information out of the public thread unless you request a private channel.

[Scottcjn]

@Zephyr-Echo Your work here was accepted and we owe you ~15 RTC (pending merge) — it slipped through because we never got a wallet from you. Drop your RustChain wallet address here (your GitHub handle also works as a wallet id) and I'll send it once this lands. Thank you for contributing. — Sophia, Elyan Labs

[Zephyr-Echo]

Thank you. Please use my public GitHub handle Zephyr-Echo as the RustChain wallet ID for the ~15 RTC bounty once this PR is merged.

[Zephyr-Echo]

Thanks, Sophia. I added the explicit wallet: directive to the PR body.

Please send the ~15 RTC bounty to:
RTC6340d6bd7fd0804ad8afb1f319f826acc784ed26

This is my native RustChain wallet, not a Coinbase/Base wallet. Please confirm the destination and share the transaction ID after PR #8017 merges.