feat(flashblocks): add canonical block reconciliation to cache and service (2)

[teddyknox]

Summary

Integrates canonical block reconciliation into the flashblock service and sequence manager:

• Adds CanonicalBlockNotification type for receiving canonical block events via channel
• Adds process_canonical_block() to SequenceManager using validators from PR Clean Documentation #1
• Service now accepts optional canonical_block_rx channel and processes notifications in the event loop
• Clears pending state on CatchUp, HandleReorg, or DepthLimitExceeded
• Adds reorg_count metric counter

New APIs

• FlashBlockService::with_canonical_block_rx() - Sets the canonical block receiver
• FlashBlockService::with_max_depth() - Configures max depth (default: 64 blocks)
• SequenceManager::process_canonical_block() - Reconciles pending state with canonical
• SequenceManager::earliest_block_number() / latest_block_number() - Query pending block range

Test plan

• Test process_canonical_block with no pending state → NoPendingState
• Test canonical catches up to pending → CatchUp, state cleared
• Test canonical behind pending → Continue, state preserved
• Test depth limit exceeded → DepthLimitExceeded, state cleared
• Test earliest_block_number / latest_block_number tracking

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.7%. Comparing base (466c2b1) to head (b1f8c96).

Additional details and impacted files

@@                        Coverage Diff                         @@
##           teddyknox/flashblock-sequencing   #18994     +/-   ##
==================================================================
+ Coverage                             79.8%    81.7%   +1.8%     
==================================================================
  Files                                  133      133             
  Lines                                 6909     6909             
==================================================================
+ Hits                                  5519     5647    +128     
+ Misses                                1390     1262    -128     

Flag	Coverage Δ
contracts-bedrock-tests	81.7% <ø> (+1.8%)	⬆️

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see 2 files with indirect coverage changes

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[einar-oplabs]

Looks good.