Add Arrow-based argument specification serialization

[rustyconover]

Summary

• Implement serialization/deserialization of function argument specifications using Apache Arrow schemas
• Enable functions to describe their argument signatures (types, positions, special markers) for IPC transmission and DuckDB function registration
• Use a single Arrow schema format where positional args come first, named args follow with metadata markers

Changes

• vgi/argument_spec.py - Core module with ArgumentSpec dataclass and serialization functions
• tests/test_argument_spec.py - Comprehensive tests (43 passing test cases)
• docs/argument-serialization.md - LLM-friendly documentation
• vgi/__init__.py - Export new public API

New Public API

from vgi import (
    ArgumentSpec,              # Dataclass for argument specifications
    argument_specs_to_schema,  # Convert specs to Arrow schema  
    schema_to_argument_specs,  # Convert schema back to specs
)

Design

Uses a single Arrow schema where:

• Field order = position index for positional args
• Named args marked with {b"vgi_arg": b"named"} metadata
• Special types use metadata: vgi_type=table, vgi_type=any, vgi_varargs=true

Test plan

• All 43 tests pass including mypy type checking
• Ruff lint and format pass
• Import verification works

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