feat(anvil): add Base proof devnet L1 APIs

[PelleKrab]

Motivation

Base's prover stack needs three things from Anvil when it serves as the single devnet L1:

• debug_getRawHeader — canonical RLP-encoded L1 headers for witness construction
• debug_getRawReceipts — canonical EIP-2718 receipt bytes for witness construction
• GET /eth/v1/config/spec — SECONDS_PER_SLOT so Base's Beacon client can map L1 execution timestamps to Beacon slots when retrieving blob sidecars from the same Anvil instance

Solution

The debug endpoints landed upstream after this fork's divergence point, so this PR includes them as verbatim upstream cherry-picks (original authorship preserved, -x annotated):

• feat(anvil): support debug raw receipts foundry-rs/foundry#15433 — debug_getRawReceipts (includes fork-mode forwarding of pre-fork blocks)
• feat(anvil): support debug raw transactions foundry-rs/foundry#15432 — debug_getRawTransactions (picked to keep the stack conflict-free with upstream)
• feat(anvil): support debug raw header foundry-rs/foundry#15430 — debug_getRawHeader

The only fork-specific commit is the Beacon spec endpoint: add GET /eth/v1/config/spec and return the positive interval-mining duration as the Beacon-compatible SECONDS_PER_SLOT string. Return an internal error when interval mining is unavailable or disabled.

This supersedes the hand-written implementations previously proposed in #60, #61, and #62, shrinking the fork's maintained surface to the spec endpoint alone; the debug endpoints will merge cleanly on the next upstream sync.

Note one semantics change relative to the superseded implementations: the debug endpoints return a BlockNotFound JSON-RPC error for unknown blocks instead of null, matching upstream.

Stack

1. fix(deps): update ruint security advisory #63 — cargo-deny security prerequisite
2. this PR — upstream debug endpoint cherry-picks + Beacon /eth/v1/config/spec

Testing

• cargo test -p anvil-core --lib
• cargo test -p anvil --test it beacon_api -- --nocapture
• cargo test -p anvil --test it -- can_get_raw
• cargo fmt --all -- --check
• git diff --check

PR Checklist

• Added Tests — upstream serde + integration tests for the debug endpoints, beacon_api test for the spec endpoint
• Added Documentation — N/A: this repo has no RPC endpoint docs surface; endpoints are documented via doc comments (the debug endpoints are covered by the upstream Foundry book)
• Breaking changes — none; additive endpoints only