feat(chain)!: Add `ApplyBlockError` for `prev_blockhash` validation

feat(chain)!: Add `ApplyBlockError` for `prev_blockhash` validation

Introduce `ApplyBlockError` enum with two variants:
- `MissingGenesis`: genesis block is missing or would be altered
- `PrevBlockhashMismatch`: block's `prev_blockhash` doesn't match expected

This replaces `MissingGenesisError` in several `LocalChain` methods:
- `from_blocks`
- `from_changeset`
- `apply_changeset`

Also adds test cases for `merge_chains` with `prev_blockhash` scenarios:
- Update displaces invalid block below point of agreement
- Update fills gap with matching `prev_blockhash`
- Cascading eviction through multiple blocks

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>