feat: add GrammarInterpreter and ImagePipe validation tests

- Add runtime interpreter for GrammarIR (runtime2/interpreter.rs)
  - Executes grammar patterns directly without code generation
  - Supports all pattern types: literals, char classes, sequences, choices,
    optionals, repeats, lookaheads, SOI/EOI
  - Handles atomic rules with text capture
  - Proper whitespace/comment skipping between elements

- Add comprehensive ImagePipe grammar tests (45 tests)
  - Grammar structure validation (metadata, builtins, rule count)
  - Pattern structure tests for all 23 ImagePipe rules
  - Terminal rule tests (identifier, integer, string_literal, etc.)
  - Interpreter parsing tests for individual statements
  - End-to-end parsing of vintage.imgpipe using actual grammar

- Fix whitespace handling bugs:
  - Preserve position 0 for SOI matching
  - Skip whitespace before first item in repeat patterns

Phase 4 of ZynPEG 2.0 plan validated against ImagePipe DSL.

Author: darmie