Refactor ErrorCode to include NodePtr and drop ValidationErr

[cameroncooper]

Note

Medium Risk
Wide, cross-cutting change to error types and propagation across consensus-critical parsing/validation code; behavior should be equivalent but regressions could surface via mismatched error construction or lost node context.

Overview
Refactors validation errors to eliminate ValidationErr and make ErrorCode carry location context. A new error_code module defines ErrorCode variants that embed NodePtr (or Option<NodePtr>) and provides CLVM list helpers (first/rest/next/atom/check_nil) plus u32 mapping and EvalErr/io conversions.

All consensus/mempool paths (condition parsing/validation, generator execution, spendbundle validation, additions/removals, message parsing, timelock checks, fingerprinting, sanitizers) are updated to return Result<_, ErrorCode> and construct errors via the new variants/closures; tests and fuzz targets are adjusted to compare by numeric code or pattern-match variants. Error::Validation now wraps ErrorCode directly (with From<ErrorCode>), and lib.rs exports error_code instead of validation_error.

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Fuzz test drops node-data comparison weakening assertion

Low Severity

The old error comparison checked both error code equality AND node data equality (node_to_bytes). The new code only compares u32 error codes via u32::from, completely dropping the node-data check. If two runs fail with the same error type but at different CLVM nodes (indicating a meaningful behavioral difference), the fuzz test will no longer detect it.