Add TIP-871: Canonicalize MODEXP output length when modulus is zero

[yanghang8612]

Adds TIP-871 (tip-871.md) at Last Call.

• What: canonicalize the MODEXP precompile (0x…05) to return modLen zero bytes for an all-zero modulus with modLen > 0, instead of empty output — aligning with EIP-198 and all major Ethereum clients.
• Gating: behind ALLOW_TVM_OSAKA (proposal id 96), shipping with the Osaka VM batch (TIP-7883 / TIP-854 / TIP-7951 / TIP-7939 / EIP-7823). No new proposal id.
• Implementation: merged in java-tron via fix(vm): canonicalize ModExp zero modulus output (TIP-871) java-tron#6780 (into release_v4.8.2), with direct Osaka MODEXP tests, a Solidity 0.8.25 VM-level reproduction, a gas regression, and frozen geth/Besu differential fixtures. Gas unchanged, cross-client output parity confirmed.
• Discussion: TIP-871: Canonicalize MODEXP output length when modulus is zero #871

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