feat: Rex4 relative gas detention cap for volatile data access

[RealiCZ]

Summary

Rex4 changes gas detention from an absolute cap to a relative cap.

• Previously, when a transaction accessed volatile data (block env, beneficiary, oracle), the compute gas limit was capped at an absolute value (e.g., 20M).
• If the transaction had already consumed >20M compute gas before the access, it halted immediately — even though the intent is to limit post-access computation, not penalize prior work.
• Now in Rex4+, the detention cap is relative to current usage at the access point: effective_limit = usage_at_access + cap. A transaction that used 25M gas before accessing TIMESTAMP gets an effective limit of 45M (25M + 20M), allowing 20M more gas after access. Pre-Rex4 specs retain the absolute-cap behavior for backward compatibility.

Changes

• ComputeGasTracker::set_detained_limit(): Computes relative cap (tx_usage + cap) for REX4+, absolute cap for pre-REX4.
• execution_result(): Detects detention-caused halts by comparing against detained_limit instead of the raw cap. Reports detained_limit in VolatileDataAccessOutOfGas.limit so actual > limit always holds.

Test plan

• cargo test -p mega-evm -- gas_detention — 5 new REX4 relative cap tests pass
• cargo test -p mega-evm — full test suite (488 tests) passes