dm: let players opt into a refresh rate below 0.5s

[dfrisone]

Summary

• Adds an Unsafe Refresh Rate toggle (cog next to Refresh Rate, off by default) that extends the slider down to 0.1s
• Off (default): unchanged, floor stays at 0.5s
• On: any value below 0.5s snaps back up immediately when toggled back off, not just on next login
• A 0.05s hard floor applies either way, so a corrupt/hand-edited SavedVariables value can never reach the ticker as 0 or negative

Why

The 0.5s floor added in v9.1.5 (allocation cost per full session snapshot per window per tick) was a hard, unconfigurable cap. A user asked to be able to go lower on a machine that can handle it. This makes it an informed opt-in instead of removing the protection outright.

Test plan

• Enable Unsafe Refresh Rate, drag Refresh Rate down to 0.1s, confirm meters refresh faster in combat
• Toggle off after setting a sub-0.5s value, confirm it snaps back to 0.5s immediately
• A different profile that never touched this setting still defaults to the 0.5s floor
• A SavedVariables value below 0.5s with the toggle off still clamps to 0.5s on login (unchanged migration)
• A SavedVariables value of 0 or negative with the toggle on clamps to the 0.05s hard floor, not 0