Add --explain flag for query plan introspection

[vmvarela]

Description

Add a --explain flag that runs EXPLAIN QUERY PLAN on the user's query and prints the SQLite execution plan to stderr before executing the actual query.

sql-pipe data.csv --explain 'SELECT region, SUM(amount) FROM t GROUP BY region'
# stderr: QUERY PLAN:
# stderr: -> Scan on t  (cost=0.00..5.00 rows=100)
# stderr: -> HashAggregate  (cost=0.50..1.00 groups=5)

Motivation

Users writing complex JOINs or multi-file queries have no way to understand performance characteristics. --explain is the single most-requested debugging feature in SQL tools and is critical for the multi-file join use case.

Acceptance Criteria

• --explain flag is parsed in args.zig
• Before executing the user query, run EXPLAIN QUERY PLAN <query> and print each row to stderr
• The actual query still executes normally and results go to stdout
• --explain is mutually exclusive with --columns, --validate, --sample, --schema, --stats
• Integration tests cover the new flag
• Help text updated in args.zig

Implementation Notes

• SQLite has EXPLAIN QUERY PLAN built in — no new dependencies needed
• Add explain: bool field to ParsedArgs in src/args.zig
• Parse --explain in the args loop (~line 442 of src/args.zig)
• Gate it in run() in src/main.zig
• Plan output goes to stderr only — must not affect exit codes or stdout