fix: bump mega-evm to 915f71e to fix missing storage slots

[flyq]

Summary

• Bump mega-evm to the latest main revision (915f71e) to pick up the fix for missing storage slots in stateless validation.
• Supersedes the intermediate bump to 47c7e3b from earlier on this branch.

Test plan

• cargo build --release succeeds with the new revision
• cargo test --workspace passes
• Validator successfully processes blocks that previously failed due to lacking slots

[claude]

nit: The PR title uses chore: but this picks up a bug fix (missing storage slots). Consider fix: bump mega-evm to 915f71e to fix missing storage slots so the squash commit message reflects the actual impact.

Otherwise LGTM — minimal diff, clear justification, test plan covers the key scenario.

[claude]

The PR currently has no labels. Since this bump picks up a fix for missing storage slots in stateless validation (a correctness bug), the bug label appears to be missing. Consider adding it to reflect that this change resolves incorrect validator behavior.