[codex] optimize reel text rebuilds

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Summary

This PR reduces unnecessary rebuild, measurement, and per-frame widget work in ReelText without changing the public API, and adds a dedicated example stress screen for visual/performance checks.

• Settled text now reuses a measured run when text, style, and layout inputs are unchanged, so parent rebuilds do not repeatedly split and measure the same glyph run.
• Rolling rows now keep the slot row outside the animation tick; the AnimatedBuilder updates viewport sizing/alignment while slots receive the animation directly.
• Changed plain-text slots paint through a lightweight render-object path that prepares glyph face layouts once and reuses them while the animation progresses.
• WidgetSpan slots intentionally stay on the widget path because they are live child widgets with layout, baseline, semantics, keys, and selection behavior.
• The example app now has a fourth Performance tab with a full-screen checkerboard of independent ReelText widgets, theme support, edge fade, optional frame logging, and a tile-count slider that starts at 4 tiles and scales up to the stress maximum.

Notes

• The temporary chroma/color toggles were removed because disabling them did not improve FPS and made the performance panel noisier.
• The reverted rolling color-face cache experiment is not part of the final effective diff.

Validation

• flutter test test/reel_text_test.dart
• cd example && flutter test test/widget_test.dart
• cd example && flutter analyze
• git diff --check

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Codex Review: Didn't find any major issues. Hooray!

Reviewed commit: 33f7d453dc

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Codex Review: Didn't find any major issues. You're on a roll.

Reviewed commit: b4b30fd99b

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