feat: expand CSV reader/writer with config, delimiters, and tests

[alvinreal]

Summary

Closes #11 — CSV reader/writer.

Changes

New functionality:

• CsvConfig struct: Configurable has_headers, delimiter, and flexible options
• from_str_with_config() / from_reader_with_config(): Parse CSV with custom settings
• to_string_with_config(): Serialize with custom delimiter and header options
• No-header mode: Returns array of arrays instead of array of maps
• Custom delimiters: Tab (TSV), pipe, semicolon, etc.
• Flexible/ragged rows: Extra columns get auto-named (column_N), short rows have missing keys
• Union headers: When serializing, all keys from all rows become columns
• Array-of-arrays output: Write raw arrays without headers

New tests (45+):

• Basic CSV parsing (single/multi row, many columns)
• No-header mode (array of arrays)
• Custom delimiters: TSV, pipe, semicolon (with roundtrip)
• Quoted fields: commas, embedded quotes, newlines (with roundtrip)
• Type inference: ints, floats, bools, negatives
• Empty fields and all-empty rows
• Cross-format roundtrips: CSV↔JSON (both directions)
• Large CSV: 1000 rows
• Unicode: Japanese headers/values, emoji roundtrip
• Ragged rows: strict error + flexible mode
• Union headers from heterogeneous rows
• Error cases: non-array input, scalar input
• Empty input/output edge cases
• from_reader with default and custom config
• Null serialization (empty field)
• Bool roundtrip
• Header order preservation
• Whitespace in quoted fields
• Array-of-arrays output and no-header roundtrip

All checks pass: cargo build, cargo test (246 tests), cargo fmt --check, cargo clippy -D warnings.