Attributed editing: paragraph-scoped rich-textarea on TextBuffer + TextSpan

Summary

Proposal for paragraph-scoped attributed editing in Native: keep UTF-8 TextBuffer as the source of truth (same IME / undo / clipboard path as textarea), attach parallel style runs, and expose <rich-textarea> that stamps WidgetRuntimeFlags.rich_editor.

Dogfooded in Tonic desktop (TONIC_NATIVE_RICH_EDITOR=1 + local NATIVE_SDK_PATH) before opening this issue — Source + Bold/Italic/Strike + GFM preview from runs; Lexical webview remains the full-GFM path.

Motivation

Today Native splits plain editable text from display-only rich spans / <markdown> render. Apps that need styled editing fall back to WebView. We want toolkit-native attributed editing without inventing a second edit engine.

v1 design (locked)

• SoT: contiguous UTF-8 TextBuffer bytes.
• Style runs: bold / italic / underline / monospace / strikethrough keyed by byte ranges (text_attr.zig ↔ @native-sdk/core/text-attr).
• Markup: <rich-textarea> → WidgetKind.textarea + runtime_flags.rich_editor.
• IME: composition range stays unstyled until commit.
• Style undo: parallel stack (pushStyleUndo / popStyleUndo); text undo stays on the buffer.
• One paragraph first (matches non-nesting span model + per-widget selection).

Capacities / a11y (from dogfood)

• Style-run budget matches max_text_spans_per_paragraph.
• Hit-test: Zig hitTestAttributed uses span layout + snap; TS has a proportional fallback until paint is wired.
• Should expose the same textbox semantics as textarea.

Non-goals for v1

Tables, mermaid, math, nested spans, cross-block selection, Lexical feature parity.

Later (v2)

Multi-block document model (text_doc.zig sketch) + GFM round-trip — separate PR series after v1 lands.

Implementation status (local branch feat/attributed-rich-textarea)

• text_attr.zig + TS twin + tests
• Schema element rich-textarea (code 71) + markup stamp + docs/skills
• examples/rich-textarea
• Tonic dogfood wiring documented in Tonic’s apps/desktop/README.md

Stacked PRs to follow once this design direction is confirmed.

Review the locked v1 design alongside text_attr.zig, its TypeScript twin and tests, the rich-textarea schema and markup stamp, and examples/rich-textarea. The local feat/attributed-rich-textarea branch already contains these pieces; the remaining outcome is confirmation of the design direction before stacked PRs follow.