feat: add --validate mode to check CSV syntax without running a query

[vmvarela]

Summary

• Adds --validate flag that parses the entire CSV/TSV input without executing any SQL query
• On success prints OK: <n> rows, <m> columns (<col> <TYPE>, ...) to stdout and exits 0
• On CSV parse error prints the existing error message (row number + description) and exits 2
• Compatible with --delimiter, --tsv, --no-type-inference
• Mutually exclusive with a query argument

Changes

• src/main.zig: new ValidateArgs struct, validate variant in ArgsResult, ValidateWithQuery error, full runValidate() implementation with type inference, thousands-separated row count, and column type summary
• build.zig: 4 new integration tests (75–78) — valid CSV, malformed CSV exits 2, custom delimiter, conflict with query
• README.md: new row in the flags table for --validate
• docs/sql-pipe.1.scd: new *--validate* entry in OPTIONS section

Acceptance Criteria

• --validate parses the entire CSV input and prints a summary to stdout on success: OK: <n> rows, <m> columns (<col> <TYPE>, ...)
• On parse error, print the existing CSV error message (row number + description) and exit 2
• --validate does not run any SQL query — no query argument required or used
• Works with --delimiter, --tsv, --no-type-inference
• Exit 0 on success, exit 2 on CSV error
• Documented in --help, README.md, and docs/sql-pipe.1.scd
• Tests: valid CSV exits 0 with correct summary; malformed CSV exits 2 with row number

Closes #88