[Core] Add if/else conditional execution

[jbatte47]

Problem

Without conditionals, Sigil cannot express branching computation and cannot progress toward a general computational core.

This issue adds first-class conditional execution.

Scope

• Add syntax, AST, and runtime support for conditional branching.
• Proposed baseline form:
  • if <expr>: <block> else: <block>
• Support condition expressions built on issue [Runtime] Achieve NODE_TRIGGER parity with member-call semantics #10 operators.

Deliverables

• Grammar productions for if/else.
• AST node(s) and runtime evaluation logic.
• Tests for true branch, false branch, and nested conditions.

Acceptance Criteria

• Conditional blocks execute correct branch based on expression result.
• Existing scripts continue to parse/execute.
• Parser conflicts are not reintroduced.

Validation Checklist

• Add parser fixtures for indentation and inline/blocked forms (if supported).
• Add runtime branch tests with deterministic outputs.
• Run full examples suite.

Non-goals

• Looping constructs.
• Advanced pattern matching.
• Module/import redesign.