Support split and burn handlers

[elazarg]

This PR significantly refactors the Vegas compiler to transition from a prototype scripting language to a structured, robust DSL for game theory. It introduces a formal ActionDag intermediate representation, explicit commit-reveal syntax, and first-class support for subgame-perfect "quit" semantics (split/burn).

Key Changes

1. Language Syntax & Design

• Explicit Structure: Games are now encapsulated in game main() { ... } blocks, allowing for future modularity.
• Commit-Reveal: Introduced commit and reveal keywords to explicitly model information hiding, replacing implicit inference.
• Quit Semantics: Added split and burn keywords to declaratively handle player abandonment, replacing error-prone manual null checks in withdraw clauses.

2. Compiler & IR (ToIR.kt, IR.kt)

• ActionDag: The compiler now lowers the AST into a dependency-based ActionDag before backend generation. This decouples game logic from execution details.
• Payoff Desugaring: Automated generation of complex "alive" predicates and timeout logic for split/burn payoffs.
• Let Elimination: Enforced removal of Let expressions during the lowering phase to simplify IR analysis.

3. Solidity/EVM Backend (GameToEvmIR.kt)

• Stateless Execution: Rewrote the EVM backend to use the ActionDag linearization. Actions are now gated by immutable ACTION_ID constants rather than mutable phase counters.
• Security: Implemented CheckReveal logic to cryptographically bind hidden values to specific roles and actions, preventing replay and copy-commit attacks.

4. Testing & Reliability

• Golden Master Testing: Added ExamplesValidationTest.kt and copy-test-outputs.py to detect regressions in generated output (Solidity, EFG, etc.) against a known good state.
• Semantics Formalization: Updated Semantics.kt to treat "Quit" as a first-class move in the game's transition system.

5. Documentation

• Added SUBGAMES.md specifying conservation of money and availability rules for subgames.
• Added AGENTS.md for AI agent context.