import-laddr: --limit semantics need a --help doc

\`--limit=N\` truncates per-table to N *imported* rows, but the report still counts pre-limit rows under \`input\`, so \`imported + skipped + errors < input\` when the flag is in play. This is intentional — dry-run reports should reflect the dump's actual size — but may surprise staff scanning a report.

Once we settle on a CLI help framework (in apps/api/scripts/), document:
- \`--limit\` precisely (per-table cap on imported rows; input still counts dump size)
- All the other flags
- The 7-commit pattern + idempotence semantics

Filed as Follow-up from PR #24 (laddr-import).