fix: Rex3 system address exemption, doc updates, and tests

[Troublor]

Summary

• Add MEGA_SYSTEM_ADDRESS exemption to the Rex3 SLOAD-based oracle gas detention path in host.rs. Without this, system transactions that read oracle storage would be incorrectly subject to gas detention.
• Update Rex3.md spec to document SLOAD-based detention semantics (caller-agnostic triggering, DELEGATECALL escape, system address exemption semantic difference from pre-Rex3 CALL-based path).
• Fix stale documentation referencing hardcoded "1M" oracle gas limits across tracker.rs, instructions.rs, mod.rs, README.md, and BLOCK_AND_TX_LIMITS.md.
• Add 8 new tests covering system address exemption, non-oracle SLOAD, nested call chains, combined block env + oracle limits, sendHint interception, and keyless deploy compute gas limit enforcement.

Changes

File	Change
crates/mega-evm/src/evm/host.rs	Add MEGA_SYSTEM_ADDRESS exemption to Rex3 SLOAD oracle detention
specs/Rex3.md	Document SLOAD semantics, fix grammar
README.md	Add 2 missing Rex3 features
crates/mega-evm/src/evm/mod.rs	Fix Rex3/Rex4 doc comments
crates/mega-evm/src/evm/instructions.rs	Fix stale 1M gas reference
crates/mega-evm/src/access/tracker.rs	Fix stale 1M gas references in examples
docs/BLOCK_AND_TX_LIMITS.md	Add SLOAD + keyless deploy to Rex3 section
crates/mega-evm/tests/rex3/system_address.rs	New: 2 system address exemption tests
crates/mega-evm/tests/rex3/oracle_gas_limit.rs	Add 4 new oracle SLOAD tests
crates/mega-evm/tests/rex3/keyless_deploy.rs	Add 1 compute gas limit test
crates/mega-evm/tests/rex3/main.rs	Add mod system_address

Test plan

• cargo check -p mega-evm passes
• cargo test -p mega-evm — all 311 tests pass (15 rex3 tests, including 8 new)
• cargo fmt --all --check — clean
• cargo clippy --workspace --lib --examples --tests --benches --all-features --locked — clean