Update EIP-7793: Clarify TXINDEX stack encoding and uint64 sentinel semantics

[Forostovec]

This commit removes ambiguity in EIP-7793 around the TXINDEX opcode result and the sentinel value used by uint64 fields:

• Define that TXINDEX pushes a 256-bit stack word with a 64-bit big-endian value zero-extended to 256 bits (left-padded to 32 bytes).
• Specify that the sentinel -1 for uint64 fields is the concrete value 0xffffffffffffffff (2^64 - 1), and when surfaced in EVM (e.g., via TXINDEX), it MUST be zero-extended to a 256-bit stack word.

Rationale:

• EVM stack items are always 256-bit words. Stating "uint64 in big endian" without explicit zero-extension leaves room for divergent implementations.
• Similar recent EIPs (e.g., SLOTNUM) use uint64, but explicit padding rules ensure consistent client behavior and avoid misinterpretation.

[eth-bot]

File EIPS/eip-7793.md

Requires 1 more reviewers from @Marchhill, @smartprogrammer93