Add --max-rows flag to limit input size

[vmvarela]

Description

sql-pipe loads the entire CSV into an in-memory SQLite database. A malformed input or an accidental pipe of a 10M-row file can exhaust RAM with no warning. A --max-rows flag gives users a safe fast-fail mechanism.

Example

$ cat huge.csv | sql-pipe --max-rows 100000 'SELECT COUNT(*) FROM t'
error: input exceeds --max-rows limit (100000 rows)

Acceptance Criteria

• Add --max-rows <n> flag (integer, must be > 0)
• After inserting n rows, stop reading stdin and exit with code 1 and a clear error message to stderr: error: input exceeds --max-rows limit (<n> rows)
• --max-rows 0 is an error (print usage error)
• Flag is documented in --help output, README.md Flags table, and docs/sql-pipe.1.scd
• Tests cover: normal operation (under limit), limit hit (correct error + exit code)

Notes

• Counter is incremented in the CSV insert loop in src/main.zig
• Exit code 1 (usage/input error) is appropriate; consider a dedicated code if it helps scripting