feat: implement chain tracker interface and WhatsOnChain

[sgbett]

Summary

Implements the chain tracker abstraction for SPV merkle root verification (HLR #93):

• ChainTracker base class — valid_root_for_height?(root, height) and current_height contract, matching all three reference SDKs ([Task 1] Core: ChainTracker base class #122)
• WhatsOnChain implementation — queries WoC /block/{height}/header endpoint, extracts merkle root, supports mainnet/testnet/STN, optional API key, injectable HTTP client ([Task 2] Implementation: WhatsOnChain chain tracker #123)
• MerklePath#verify — computes merkle root from path + txid, validates against chain tracker, completing the SPV verification chain ([Task 3] Integration: MerklePath#verify with chain tracker #124)
• Conformance specs — 16 specs cross-validating against Go SDK test patterns, genesis block data, end-to-end SPV verification, and URL construction ([Task 4] Testing: Chain tracker conformance specs #125)

601 lines added across 9 files. Full test suite: 1400 examples, 0 failures.

Closes #93, closes #122, closes #123, closes #124, closes #125

Test plan

• 5 ChainTracker base class specs pass
• 13 WhatsOnChain unit specs pass (mocked HTTP)
• 3 MerklePath#verify specs pass
• 16 conformance specs pass (Go SDK patterns, SPV end-to-end)
• Full suite: 1400 examples, 0 failures
• RuboCop clean on all changed files

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 96.29630% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...b/bsv/transaction/chain_trackers/whats_on_chain.rb	94.73%	2 Missing ⚠️

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