Replace sqlparser-rs with DuckDB native parser

[nicosuave]

Summary

• Replace sqlparser-rs with DuckDB's native Parser class via C++ FFI
• Add support for all DuckDB aggregate functions (STDDEV, MEDIAN, STRING_AGG, etc.)
• Detect and reject non-decomposable aggregates (COUNT DISTINCT, MEDIAN, PERCENTILE) with clear error messages at query time
• Remove "experimental" label from README

Changes

• C++ FFI layer: New yardstick_parser_ffi.cpp uses DuckDB's Parser directly
• Rust bindings: parser_ffi.rs provides safe Rust wrappers
• Removed sqlparser-rs: No longer a dependency
• Non-decomposable detection: Functions like COUNT(DISTINCT) fail with helpful error when used with AGGREGATE()
• Extended tests: DuckDB-specific functions (DATE_TRUNC, COALESCE, array functions, cast syntax)

Non-decomposable Aggregates

These aggregates work in views but cannot be used with AGGREGATE():

-- Works: direct query
SELECT region, unique_customers FROM v;

-- Fails with clear error: cannot re-aggregate
SEMANTIC SELECT region, AGGREGATE(unique_customers) FROM v;