fix: remove SELFDESTRUCT from key differences table; correct sequencer failover latency

[Troublor]

Summary

• Removes the SELFDESTRUCT row from the "Key Differences at a Glance" table in docs/dev/execution/overview.md. EIP-6780 semantics are standard Ethereum behavior post-Dencun and are not a MegaETH-specific difference. Also updates the frontmatter description to drop the stale reference.
• Corrects the sequencer failover latency claim in docs/architecture.md from "within seconds" to "within tens of milliseconds" to accurately reflect the hot-backup capability.

Fixes #29
Fixes #30

[claude]

Reviewed. Both changes are correct and the diff is clean — no issues.

[claude]

The table row was removed on the grounds that EIP-6780 semantics are standard post-Dencun Ethereum behaviour, not a MegaETH-specific difference. The same reasoning applies to the dedicated ## SELFDESTRUCT with EIP-6780 Semantics section that remains at lines 52–57 of docs/dev/execution/overview.md — it documents standard Ethereum behaviour, not a MegaETH deviation, so it should be removed for consistency.

If the spec page at https://docs.megaeth.com/spec/megaevm/selfdestruct covers a MegaETH-specific nuance beyond plain EIP-6780, that nuance should be surfaced explicitly. Otherwise the section (and the spec link) should go.