Feature: :TIMING ON/OFF command for interactive sessions

Summary

Add a :TIMING command to toggle performance timing display on/off during an interactive session.

Use Case

• Enable/disable timing without restarting sqlcmd
• Temporarily check performance for specific queries
• Better interactive debugging workflow

Proposed Syntax

:TIMING ON
SELECT * FROM users
GO
-- Shows timing info

:TIMING OFF  
SELECT * FROM orders
GO
-- No timing info shown

Relationship to Existing Features

• Acts as a runtime toggle for what -p flag enables at startup
• Complements :PERFTRACE which redirects timing output to a file
• -p flag would set initial state, :TIMING allows changing it

Implementation Notes

• Simple boolean toggle in the Sqlcmd struct
• Modify GO command to check the toggle state
• Minimal code change - leverages existing timing infrastructure

Locate the Sqlcmd struct, the existing -p timing setup, the GO command handling, and the :PERFTRACE implementation. Trace how timing is currently displayed, then add the interactive toggle so ON and OFF control subsequent queries without changing the startup behavior; verify both states in an interactive session.