fix(textinput): correct placeholder layout constraints (px vs DIP) and no-op NaN fontSize guard

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Problem

The native placeholder renders at a different height/size than typed text; a placeholder with no explicit fontSize gets a NaN size that is never repaired.

Root cause

In CreatePlaceholderLayout(): (1) LayoutConstraints are in DIPs, but the code feeds m_imgWidth/m_imgHeight, which are physical pixels (frame * pointScaleFactor) — the placeholder is laid out in a box pointScaleFactor× too large. (2) The NaN-fontSize guard is a no-op: TextAttributes::defaultTextAttributes().fontSize; is a discarded expression (missing assignment), so NaN is never replaced.

Fix

Divide the constraint by pointScaleFactor to get DIPs; actually assign the default into textAttributes.fontSize.

Validation (honest)

Probed in a production RNW 0.83.2 new-arch app (Facilitron FIT, Windows 11 ARM64, Debug, 250% scale) with stock deep-import TextInputs and no app shims:

Findings stated plainly: the fontSize no-op is an unambiguous code bug (discarded expression), though on 0.83.2 a no-fontSize placeholder still renders at a plausible default — so that part is correctness/robustness, not a visible-crash fix. At this probe geometry (44-DIP box, fontSize 18) placeholder and typed text measured pixel-identical glyph rows (both parked low — dominated by the GetContentSize centering bug, companion PR #), so the oversized constraint did not produce a visible offset in this configuration; the mismatch that motivated the fix was observed in product forms before we suppressed native placeholders app-wide. The DIP conversion mirrors how m_imgWidth/m_imgHeight are derived (verified in 0.83.2 source). Needs upstream CI + placeholder-vs-typed comparison at >100% scale across box/font sizes, with and without explicit placeholder fontSize. Authored against 0.83-stable; happy to re-cut onto main.

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Compiled into a framework source build (binary-gated) and re-probed in our production app: no regression; the NaN-fontSize guard now actually assigns (a no-fontSize placeholder renders at the default size), and placeholder-vs-typed remain pixel-comparable at this geometry. After-shot: