Add Design Doc for Composable Text Plugins in Flutter

[hannah-hyj]

Demo app: https://hannah-hyj.github.io/flutter_text_plugin_demo/
issue: flutter/flutter#65504
Prototype Branch: hannah-hyj/flutter@text-plugin-testing
#25 was closed in favor of this one

Summary

Proposes a composable text plugin architecture (TextPluginScope, TextPlugin, and TextDelegate) in package:flutter/rendering.dart and package:flutter/widgets.dart.

This enables cross-cutting text inspection, background/foreground decoration painting, pointer gesture handling, and scroll-to-range across standard Text, RichText, and EditableText (TextField) subtrees (RenderParagraph and RenderEditable) without requiring custom text widgets or TextSpan wrappers.

Key Use Cases

• In-page find & highlight (Ctrl+F) across lazy scrollables (ListView.builder) and text fields
• Non-intrusive inline annotations, spellcheck/grammar squiggles, and interactive token tooltips
• Accessibility and localization auditing overlays (e.g., text truncation, contrast, and scaling diagnostics)

Pre-launch Checklist

• I read RFC 000.0001: Taxonomy and followed the file and path naming and metadata standards.
• I read RFC 000.0002: Process and confirmed this proposal meets the threshold for a full RFC.
• I read and agree to the Code of Conduct.
• I signed the CLA.
• I have linked an issue from flutter/flutter with the label design doc.
• All existing and new tests are passing.
• I have enabled "Allow edits from maintainers" on this PR so the bot can automatically assign an RFC number (or I will run dart run bin/assign_rfc_number.dart locally when instructed).

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