feat(text-editor): add an outline to text layers and scale shadows with the text

[hm21]

Description

Text over video has to stay readable over whatever is behind it. Today the only way to separate text from a busy or bright background is a background box, which covers the picture. This adds an outline to text layers and makes their shadows behave, so text reads well without a box.

Outline

• TextLayer.outlineWidth and TextLayer.outlineColor (default: no outline), serialized in import/export with minified keys ow / oc.
• RoundedBackgroundText draws the outline as a stroke pass under the glyphs. Half of the stroke lies inside the glyph, so outlineWidth is the visible thickness outside the glyph edge.
• TextEditorState.setOutline({width, color}), initialised from the edited layer and written onto the layer done() returns. The input field previews it while typing.

Shadows

• With an outline, the shadows are cast by the outlined glyphs. A text shadow on the stroke pass would only follow the bare glyph shape, so the painter draws an opaque silhouette (stroke + fill) into its own layer, tints it with the shadow color and blurs it. Merging both passes in one layer keeps a translucent shadow color from doubling where they overlap.
• Behaviour change: shadows of textStyle (and the outline) are measured at a scale and fontScale of 1 and grow with the rendered font size. Before, pinching a text layer or raising its font scale kept the unscaled shadow offset and blur, so the shadow looked thinner the larger the text got. Unscaled layers look exactly as before.
• With TextEditorStyle.inputShadows: null the input previews the selected style's shadows at the size the layer draws them.

Fixes the feature depends on

• A text layer reserves room for its outline and shadows (reserveEffectSpace), symmetrically so its center stays put. A layer capture reads the repaint boundary's bounds, so a shadow below the glyphs was cut off in exports.
• LayerCopyManager.createCopyTextLayer and LayerInteractionManager._copyLayer list every field by hand and dropped the outline, which lost it when a layer is edited, duplicated, grouped or transformed by crop/rotate.
• TextLayer.toMapFromReference wrote shadows only while they were non-empty and even when unchanged. The import merges each history step over the previous one, so a shadow removed in a later step came back after export/import. It now writes shadows only when they changed, including an empty list.
• RoundedBackgroundTextPainter.shouldRepaint also compares the text and the outline, so a style-only change repaints.

Related Issue: none (needed for readable text and captions in a video editor built on this package).

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore

Tests

• TextLayer: outline defaults, map round trip, copyWith, and the reference diff (unchanged shadows omitted, removed shadows written as an empty list, outline changes written).
• Import/export: an outlined, shadowed layer whose shadows are removed in a later history step imports without them and keeps its outline. Fails against stable because the removed shadows come back.
• RoundedBackgroundText: one paint pass without an outline, the outline pass under the text, the silhouette shadow layer, a transparent outline drawing nothing, and the reserved space growing by exactly the outline and shadow reach.
• LayerWidgetTextItem: outline and shadows scale with scale * fontScale.
• TextEditor: setOutline reaches the returned layer; editing a layer previews its outline at the current font scale.
• Layer copies keep the outline (copy manager and grouping). Both fail without the fix.
• flutter test (695 tests), flutter analyze and dart format pass.
• Checked on a Samsung Galaxy S26 (Android) in a video editor using this branch: live preview while typing, font change, re-editing a layer, and exported videos with outline and shadow on titles and burned-in captions.