feat(script): add flow control and bitwise opcodes

[sgbett]

Summary

• Implement conditional execution: OP_IF, OP_NOTIF, OP_ELSE, OP_ENDIF with correct nested branch tracking (checks all cond_stack entries, not just top)
• Add OP_VERIFY, OP_RETURN (after-genesis early success), OP_NOP/NOP1-10 (CLTV/CSV as NOPs)
• Handle reserved opcodes (OP_VER, OP_RESERVED, OP_RESERVED1/2) and always-illegal opcodes (OP_VERIF, OP_VERNOTIF — silent in non-executing branches after genesis)
• Add bitwise operations: OP_EQUAL, OP_EQUALVERIFY, OP_AND, OP_OR, OP_XOR, OP_INVERT
• Only one OP_ELSE per OP_IF (after-genesis semantics)

Closes #59

Test plan

• 43 new examples across 2 spec files (flow_control, bitwise)
• Nested conditional tests (IF inside false branch, IF inside ELSE branch)
• RuboCop clean (0 offences across all 16 interpreter files)
• Full suite passes (810 examples, 0 failures)

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
lib/bsv/script/interpreter/interpreter.rb	83.33%	4 Missing ⚠️
.../bsv/script/interpreter/operations/flow_control.rb	97.50%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!