test: real-chain fixture for reorg integration testing

[fpelliccioni]

Start of the end-to-end harness. The reorg path spans the header index, the flat-file block store, the undo files and UTXO-Z, and its bugs live in how those agree with each other — which is why they kept surfacing in review instead of in CI.

What this adds

A fixture that brings up an actual block_chain on a throwaway directory, following the node's own startup sequence: create the store seeded with genesis → start the chain → materialize the active chain through the organizer. Regtest is deliberate: its PoW target is trivial, so competing chains can be built without mining.

It already found something

The first test pinned an ordering dependency I introduced in #572 and had not noticed:

The active chain (height → index) is materialized by header_organizer::sync_tip(), not by block_chain::start(). Until it runs, active_at() answers null_index for every height — so fetch_blocks_raw and disconnect_block, which both now resolve through it, would find nothing.

The node does this at startup so production is fine, but anything else standing up a chain must too. That is now encoded in the fixture rather than being tribal knowledge.

Scope

Fixture only. Building competing chains over it and asserting the disconnect → switch → re-download cycle is the next step, and it is the one that actually validates #571/#572/#573.

I would rather land this small and honest than describe it as more than it is: it does not yet test a reorg.

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📥 Commits

Reviewing files that changed from the base of the PR and between 67cdf68 and 0847143.

📒 Files selected for processing (3)

• src/blockchain/CMakeLists.txt
• src/blockchain/test/reorg_chain_fixture.hpp
• src/blockchain/test/reorg_e2e.cpp

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