refactor: refactor register machine into library with CLI, tests, and examples

[milesapnash]

• Restructure project as a Cabal library + executable (rm-sim) with proper module hierarchy (Pairing, Parser, Programs, RegisterMachine)
• Add instruction parser (Parser.hs) supporting the L0: R1- -> L1, L2 format, with Show/parse roundtrip
• Add executeWithLimit to cap execution steps and warn on non-termination
• Replace partial read calls in CLI argument parsing with readMaybe and clear error messages
• Add QuickCheck test suite covering pairing roundtrips, instruction/program encoding, parser roundtrips, and execution semantics
• Add 8 example programs: addition, subtraction, multiplication, copy, double, swap, equality, is_zero
• Add CI workflow (GHC 9.6, cabal build + test)
• Add README with usage docs, program format spec, and examples table
• Add MIT license and .gitignore