make fast forward a bit stricted

[arvidn]

Note

Medium Risk
Tightens consensus validation for singleton fast-forward; callers that relied on amount changes during fast-forward will now fail, but the change protects mempool cost invariants.

Overview
Singleton fast-forward now rejects spends where the rewritten lineage proof or solution would use different amounts than the target parent/coin, so serialized spend size and run cost stay unchanged.

fast_forward_singleton adds a check that lineage_proof.parent_amount matches new_parent.amount and new_solution.amount matches new_coin.amount before updating those fields (both are variable-size in CLVM). Integration tests no longer vary parent/coin amounts; they always use the original spend amount, and test_amount_mismatch covers mismatches on the new coin and new parent as well as the spent coin.

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[coveralls-official]

Coverage Report for CI Build 30468228464

Coverage increased (+0.01%) to 81.829%

Details

• Coverage increased (+0.01%) from the base build.
• Patch coverage: 16 of 16 lines across 1 file are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	19900
Covered Lines:	16284
Line Coverage:	81.83%
Coverage Strength:	11661514.64 hits per line

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💛 - Coveralls