chore(rn-tester): add Playground reproducer for Android Fabric shared TextPaint typeface leak

[pangziqiang]

Summary

Adds an RNTester Playground reproducer for #57950.

On Android with the new architecture, Fabric measures every <Text> with one shared, thread-local
TextPaint. For text without an explicit font family/weight/style, TextLayoutManager.updateTextPaint()
restores the default with paint.reset(); paint.setTypeface(null). On some OEM ROMs that does not
restore the typeface — Paint.reset() keeps the resolved font and the following
Paint.setTypeface(null) is a no-op — so the paint keeps the last typeface that was set explicitly,
and every following plain <Text> is measured with it while ReactTextView renders with the real
system font. Line boxes come out ~24% too short and descenders / trailing glyphs are clipped.

Changelog:

[Internal] - Add an RNTester Playground reproducer for the Android Fabric shared TextPaint typeface leak (#57950)

How the reproducer works

No bundled font asset is needed: any explicit font family poisons the shared paint. The screen uses
the framework alias casual (Dancing Script, present since API 21) because its vertical metrics are
far from the default font's on every Android version, so the rows are trivially distinguishable:

• R1: <Text style={{fontSize: 48, fontFamily: 'casual'}}>
• R2/R3: plain <Text style={{fontSize: 48}}> — different strings from R1

R2/R3 must use different strings: RN caches text layouts per attributed string, and an identical
string silently reuses R1's layout (our first attempt had exactly this false negative).

Verdict: R2/R3 == R1 → leaked (reproduced); R2/R3 == default-font metrics → not affected.

Test Plan

Open the Playground example in RNTester and compare the metrics printed under the three rows.
onTextLayout metrics at fontSize 48, in px (ascender / descender / height):

device / ROM	build	R1 (casual)	R2 (plain)	R3 (plain)	result
Xiaomi 17 Max, HyperOS	0.78.3 unpatched	49.7/23.7/73.3	49.7/23.7/73.3	49.7/23.7/73.3	reproduced
Xiaomi 17 Max, HyperOS	with #58036 backported	49.7/23.7/73.3	50.3/13.7/64.0	50.3/13.7/64.0	fixed
Xiaomi Pad 4 Plus, stock AOSP 16	0.78.3 unpatched	49.5/23.5/73.0	45.0/14.0/59.0	45.0/14.0/59.0	not affected

Fix: #58036.

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[pangziqiang]

信件已经收到。

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