EIP-8198: make Amsterdam slot-duration agnostic

[chugarchugarr]

Reference-spec proof that EIP-8198 can support repeated slot-duration changes without adding another fork-specific execution path.

Acceptance target:

• preserve current 12s -> 10s behavior;
• add a synthetic 10s -> 8s era as schedule data only;
• key gas-limit transitions to parent/current execution-payload duration eras;
• keep base-fee responsiveness invariant per wall-clock time using current/base duration;
• derive blob capacity and blob-fee response from the same schedule;
• make BLOBBASEFEE consume the current era;
• cover missed execution-payload slots across a duration boundary.

This PR targets an isolated base branch pinned to Barnabé Monnot's EIP-8198 execution reference commit 0e8cc7f so CI and review see only the slot-time-independence delta.

[chugarchugarr]

Superseded by #4, whose final exact-head proof is green: https://github.com/chugarchugarr/execution-specs/actions/runs/33028230600

Closing this earlier version without deleting its branch or history. The active public record is chugarchugarr/-x402-resolution-receipt#2.