feat: seal verification + Heimdall span cache in BorConsensus

[VAIBHAVJINDAL3012]

Summary

• Add compute_seal_hash() function that computes keccak256(RLP(header)) with the 65-byte seal stripped from extra data, matching Go Bor's SealHash behavior
• Enhance BorConsensus with Mutex<SpanCache> and Mutex<Recents> for thread-safe validator set lookups and anti-double-sign tracking during block validation
• Wire full seal verification into validate_block_pre_execution: ecrecover signer from seal, verify against authorized validator set from cached Heimdall spans, enforce anti-double-sign window
• Expose insert_span() for eager span cache population from Heimdall before block validation
• Add Debug derive to SpanCache

Architecture

• Header validation (validate_header): structural checks only (sync-safe, no external state needed)
• Block pre-execution (validate_block_pre_execution): full seal verification using cached spans
• Span cache is populated eagerly by an external component before blocks reach consensus validation
• If a span is not cached, validation logs a warning and skips signer authorization (graceful degradation during initial sync)

Test plan

• All 201 tests pass across bor-consensus + heimdall-client
• New compute_seal_hash determinism test
• New seal hash round-trip test (sign → compute_seal_hash → ecrecover → verify signer)
• Full workspace cargo check passes

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