Closes #66

Summary

Adds a complete Sudoku game as a Soroban smart contract, following the same ECS patterns established by the Reversi example.

• Components — BoardComponent (81-cell Vec), FixedCellsComponent (immutable given cells), GameStatusComponent,
  MoveCountComponent, all stored under a single WORLD key as ECSWorldState
• Systems — InputValidationSystem (bounds + editability + range), PlacementValidationSystem (row/col/block constraints),
  BoardUpdateSystem, CompletionSystem / EndConditionSystem
• Contract API — init_game, submit_value, get_state, get_cell, is_solved; panics on any constraint violation so invalid
  transactions are cleanly rejected by the chain
• Tests — 15 unit tests covering initialisation, move validation, constraint rejection, and full 51-move solution
• Docs — README with ECS design, API reference, and usage guide; MATCH_LOG.md with the real testnet deployment
  (CCF5MENLO56M4S72BF4H5KMU7XYO45VTB7GJ2O3BZFG7EWY6X3X2DBQO), solved in 51 moves; scripts/play.sh to build, deploy, and solve
  end-to-end on testnet

Test plan

• cargo test passes in examples/sudoku/
• cargo clippy --all-targets --all-features -- -D warnings clean
• cargo fmt --check clean
• stellar contract build produces sudoku.wasm
• ./scripts/play.sh deploys and solves the puzzle on testnet (is_solved returns true)