MudItem: Build only the breakpoint classes that are set

[danielchalmers]

Problem

Every breakpoint class was passed to AddClass as an interpolated string with its condition as a second argument:

.AddClass($"mud-grid-item-xs-{xs}", xs != 0)

Arguments are evaluated before the call, so the string is formatted whether or not the condition holds. Most items set one breakpoint out of six, so five are built and thrown away on every render.

Fix

Build each class inside its condition. The markup is unchanged.

Numbers

200 MudItem with only xs set, allocation measured on a bare Renderer with GC.GetTotalAllocatedBytes, median of 5:

	before	after
mount	478 KB	416 KB
re-render	177 KB	115 KB

That is 310 B less per item on re-render, which is the repeated cost.

Scope

Only MudItem, deliberately. The same pattern appears elsewhere, but measuring it showed the saving comes from formatting a value into the string, not from the interpolation itself, so it only pays where a number or enum is formatted into a class that usually is not used:

• MudStack has five conditional interpolated classes and the same change made no measurable difference at all (391 KB and 108 KB either way), because its values are null when unset and a null appends nothing.
• MudInputCssHelper likewise: 1026 KB against 1025 KB across a 20 field form.
• MudIconButton has six such classes, but at ~9.9 KB per instance the saving would be under 4%.

MudItem stands out because it is a very small component whose class chain is most of what it does, and because six integer breakpoints are formatted where at most a couple are used.

Verification

Class output is identical for none, xs, xs+md, all six breakpoints, xxl only, xs+Class, and Class only.

The Grid documentation page renders 30 grid items with an identical class string on this branch and on dev (same hash, same 1053 characters), before and after moving the interactive sliders.

361 Grid tests and the full suite pass.