Support dictionary unpacking in dict literals ({**x, 'key': val})

## Description Dictionary unpacking inside dict literals (PEP 448) is not yet supported by Monty's syntax parser. Attempting to use `{**expr, ...}` raises a `MontyRuntimeError` at parse time (before `.run()` is called). ## Reproduction ```python import pydantic_monty code = """ def process(result): return {**result, 'value': 'modified'} """ monty = pydantic_monty.Monty(code, inputs=["__arg0"], type_check=False) # Raises: MontyRuntimeError: NotImplementedError: The monty syntax parser does not yet support dictionary unpacking in literals ``` ## Error ``` pydantic_monty.MontyRuntimeError: NotImplementedError: The monty syntax parser does not yet support dictionary unpacking in literals ``` Note: the error is thrown at `Monty(...)` construction time, not at `.run()` — so it is a parse/AST-walk failure, not a runtime one. ## Expected behaviour `{**result, 'value': 'modified'}` should execute and return a merged dict, equivalent to: ```python {**result, 'value': 'modified'} # => {'score': 0.8, 'value': 'modified', 'confidence': 0.9} ``` ## Workaround Use explicit key construction instead: ```python {'score': result.get('score'), 'value': 'modified', 'confidence': result.get('confidence')} ``` ## Environment - `pydantic-monty` version: 0.0.7 - Python: 3.13 - Platform: macOS (darwin) ## Notes The error message already says "does not **yet** support", so this appears to be a known gap on the roadmap. Logging this to make it trackable.