feat: add suppressive captures (@_ syntax)

[zharinov]

Summary

Adds suppressive captures (@_ and @_name syntax) for matching patterns structurally without contributing to output.

Changes

• Parser: New SuppressiveCapture token (@_[a-zA-Z0-9_]*) with is_suppressive() AST method
• Type inference: Suppressive captures return TypeFlow::Void (no fields created)
• Bytecode: SuppressBegin/SuppressEnd opcodes bracket suppressed regions
• VM: suppress_depth counter skips effect emission when > 0, restored on backtrack
• Trace: Shows ⬥ for entering/exiting suppression, ⬦ for suppressed effects

Example

Func = (function_declaration
  name: (identifier) @name
  parameters: (formal_parameters) @_   ; match but don't capture
  body: (statement_block) @body)

Output only includes name and body, not the parameters.

Testing

• 5 new engine tests covering nested, sibling, and definition-call scenarios
• 6 new lexer tests for token recognition
• All 722 tests pass