Add Admin › Tests › Input Sizing test page

[amcclain]

XH-facing diagnostic harness supporting the hoist-react input-sizing audit (xh/hoist-react#4287 and related tickets #3300, #4085, #2421). Lives alongside the other admin test panels - not shipped in the user-facing Toolbox app.

What it does

• Renders a specimen per layout-capable desktop input (12 total: text/number/textArea/select/picker x2/dateInput/slider/segmentedControl/buttonGroupInput/codeInput/jsonInput) inside a configurable parent container (block / hbox / vbox, adjustable width / height / gap).
• Sidebar controls for width, flex, minWidth, maxWidth applied uniformly to every specimen, so the same scenario can be compared across inputs at a glance.
• Live per-specimen measurement: container content width vs. the input's rendered outer width, with a red mismatch indicator when a fill-expected prop (flex, width: '100%', width: null + CSS override) was set but the input didn't fill.
• Free-form CSS override editor (codeInput, mode: 'css') whose rules auto-wrap in a .tb-input-sizing-test__specimen-container { ... } CSS-nesting block, so selectors only affect the specimens. Lets a developer type the exact CSS they'd try in their own app and see who stretches vs. clips.

What it already surfaces

With flex: 1 applied to every specimen in an hbox container:

• Fill correctly: textInput, textArea, select, slider, buttonGroupInput, codeInput, jsonInput
• Fail (short): numberInput, picker (single + multi), dateInput, segmentedControl

New finding beyond the tickets: Blueprint's NumericInput forwards style to the inner <input>, not the outer .bp6-numeric-input div - a third flavor of the "wrong element" problem the audit will address.

Companion PR

Requires the cssMode opt-in file landed in xh/hoist-react#4346.