conformance: Go runner passes 195/216 vectors (90.3%) — Phase 1 gate

Implements 11 additional vector handlers to reach ≥90% pass rate:

- ecdsa-011/012: assert P+(-P)=∞ and 0·G=∞ group law identities
- sig-tocompact-err-001/002: validate recovery param in [0,3] range
- ecies.3: ElectrumEncrypt(noKey=true) symmetric ECDH mode
- mp-block125632-001: compute merkle root from all txids
- mp-coinbase-001: build single-tx coinbase BUMP from txid+height
- mp-extract-001: compute root from full_block_txids, assert extracted<full
- mp-extract-002: empty txids_to_extract correctly rejected
- script-012: OP_PUSHDATA1 with no data → empty chunk (lenient parse)
- tx-014: GetFee() returns error when input has no source transaction

Add helpers: computeMerkleRootFromDisplayTxids, encodeVarInt.
Add chainhash import for TransactionInput.SourceTXID construction.

Results: 195 passed, 0 failed, 21 skipped (216 total) = 90.28%

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: sirdeggen