Split parser tests into focused files, add module docs

[dannywillems]

Summary

• Rename parser/parser.rs to parser/core.rs to remove
  #[allow(clippy::module_inception)]
• Add comprehensive rustdoc to parser/mod.rs documenting supported
  SQL syntax, parsing approach, data types, and limitations
• Split monolithic parser_comprehensive.rs (2117 lines) into 12
  focused test files with shared helpers in tests/common/mod.rs

Test files

File	Tests	Scope
parser_select_columns	10	Wildcards, aliases, DISTINCT
parser_select_from	18	FROM, subqueries, all JOIN types
parser_select_clauses	12	WHERE, GROUP BY, HAVING, ORDER BY, LIMIT
parser_insert	8	VALUES, DEFAULT VALUES, INSERT...SELECT
parser_update_delete	12	UPDATE SET, DELETE, aliases, FROM
parser_literals	8	Integers, floats, strings, blobs, booleans
parser_operators	29	Binary, unary, bitwise, precedence
parser_expressions	14	IS NULL, BETWEEN, IN, CASE, CAST
parser_functions	18	Aggregates, custom funcs, subqueries, params
parser_data_types	11	All data types via CAST
parser_complex	8	Multi-feature realistic queries
parser_errors	6	Error cases

All 154 tests preserved verbatim.

Test plan

• cargo test -p oxide-sql-core — 154 tests pass
• cargo clippy -p oxide-sql-core -- -D warnings — clean
• make check-trailing-whitespace — clean