Support fork-aware Beacon blob reads

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Allow production rollup nodes to use a private Ethereum L1 fork without mixing local batches with the upstream continuation.

• Add an opt-in historical Beacon endpoint with stable genesis and slot metadata.
• Resolve numeric Beacon slots to exact canonical local blocks and bound historical reads at the execution fork.
• Preserve blob filtering and JSON/SSZ responses, with bounded upstream failures distinct from missing slots.
• Keep local mining slot-aligned and reject post-fork upstream block-by-hash results.
• Preserve the slot identity through reset and document the L1-only workflow.

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Validation: the Beacon tests passed in CI; 259 selected offline Anvil tests passed locally, along with Clippy and formatting.

The CI run is blocked outside this diff:

• The unchanged lockfile contains rustls 0.23.43, flagged by RUSTSEC-2026-0285.
• Five Forge CLI tests fail on fixture/output or external-RPC behavior, including a forge-std revision mismatch and a Lisk RPC TLS handshake failure.
• The external snekmate fixture is missing OpenZeppelin sources after its dependency installation.

No checks were disabled and no snapshot data was modified. These repository/dependency maintenance issues are not bundled into the Beacon change.

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Smaller draft replacements are now available: #77–#85 cover filter validation, fee history, state locking, slot time, upstream reads, routing and atomic reset. The complete qualification tip and dependency map are in #88.

Keeping this draft and its branch intact while replacement CI and the Base integration pin are unresolved. No existing revision or running experiment was changed.

Comment generated by Amp.