feat(flashblocks): Extract Validation & Sequencing Logic from StateProcessor

[refcell]

Summary

Extract pure validation logic from StateProcessor into dedicated, unit-testable types.

This is a first pass at incremental refactoring to improve flashblocks testability and legibility.

Problem

Flashblock validation logic is embedded in StateProcessor::process_flashblock() and is_next_flashblock(), making it impossible to unit test without the full integration harness.

Solution

Create src/validation.rs with three pure types:

1. FlashblockSequenceValidator - Validates flashblock ordering

pub enum FlashblockValidation {
    NextInBlock,
    FirstOfNextBlock,
    Duplicate,
    NonSequentialGap { expected: u64, got: u64 },
    NewBlockNonZeroIndex { block_number: u64, index: u64 },
}

2. ReorgDetector - Detects chain reorgs by comparing tx sets

pub enum ReorgDetection {
    NoReorg,
    Reorg { expected_count: usize, actual_count: usize },
}

3. ReconciliationStrategy - Explicit handling strategies for canonical blocks

pub enum ReconciliationStrategy {
    CatchUp { canonical_block: u64 },
    Reorg { from_block: u64 },
    DepthExceeded { depth: u64, max_depth: u64 },
    Continue,
}

Acceptance

• Unit tests for all validation states
• Integration tests still pass
• No public API changes