test: expand ZynML e2e tests with comprehensive spec coverage

Rewrote and expanded the e2e_tests.rs to comprehensively test ZynML
language features against the unified ML DSL specification in
docs/ml-dsl-plans/00-unified-ml-dsl.md.

Test coverage now includes:
- Grammar compilation (LanguageGrammar and Grammar2)
- Module and import system
- Type system (structs, enums, type aliases, abstract types, traits,
  impl blocks, opaque types, generics with bounds)
- Function definitions (simple, with params, generic, expression-bodied)
- All statements (let, assignment, if/else, while, for, return,
  break, continue)
- All expressions (arithmetic, comparison, logical, unary, matrix
  multiply @, pipe |>, ternary, ranges, calls, method chains, indexing,
  member access, path expressions, extern calls)
- All literals (int, float, string, bool, array, tensor, struct,
  duration/suffixed)
- Data loading, pipeline, compute/kernel, visualization, streaming,
  caching, and config syntax (with documentation of pending features)
- Example files and stdlib parsing
- Runtime and execution tests
- Parse error handling

Also includes a spec compliance summary test that documents:
- 30+ implemented language features
- 14 pending spec features (module declarations, aliased imports,
  pipeline keyword, render statements, etc.)

114 tests, all passing.

Author: darmie