feat(mix): complete widget modifier config APIs

[leoafarias]

Related issue

None. Implements the proposal from #1053, which this PR supersedes.

Description

Every built-in modifier now has both a WidgetModifierConfig factory and a matching fluent chain method, so callers no longer need .modifier(SomeModifierMix(...)) to reach a first-party modifier. modifier / modifiers remain the entry points for custom modifiers.

The audit that motivated this found two kinds of gap:

API	Factory	Chain method	This PR
mouseCursor	missing	missing	adds both
scrollView	missing	missing	adds both
skew	exists	missing	adds chain method
defaultIcon	exists	missing	adds chain method
iconTheme	exists	missing	adds chain method
box	exists	missing	adds chain method
reset	exists	missing	adds chain method

Purely additive. Every new member delegates to an existing factory and merge, so modifier ordering, merge keys, null/default handling, rendering, and animation behavior are unchanged. No existing signature changed and no consumer migration is needed.

final style = BoxStyler().wrap(.mouseCursor(SystemMouseCursors.click));

final wrappers = WidgetModifierConfig.opacity(0.8)
    .mouseCursor(SystemMouseCursors.click)
    .scrollView(scrollDirection: Axis.horizontal);

A note on reset()

The chained reset() is merge(WidgetModifierConfig.reset()), which collapses to an empty modifier list rather than retaining the reset marker. So it clears the modifiers accumulated in that configuration only — merging the result into another configuration no longer clears that one:

base.merge(other.reset());                      // base's modifiers survive
base.merge(WidgetModifierConfig.reset());       // base's modifiers are cleared

That is pre-existing merge behavior, not something introduced here. It is characterized in tests and documented on the method, with a pointer to the factory for the merge-surviving case. Redesigning the reset algorithm was deliberately left out of scope.

Changes

• packages/mix/lib/src/modifiers/widget_modifier_config.dart: mouseCursor and scrollView factories + chain methods; chain methods for skew, defaultIcon, iconTheme, box, reset. Dartdoc on the new entry points, with the reset() semantics spelled out.
• packages/mix/test/src/modifiers/widget_modifier_config_test.dart (new, 25 tests): factory ↔ explicit-modifier parity; full option forwarding for scrollView and iconTheme including omitted-stays-null; chain ↔ merge parity; contextual dot shorthand through a Styler's wrap; same-type merge without duplication; source-configuration immutability; custom orderOfModifiers surviving chaining; six reset characterization tests. Two widget pumps assert the real MouseRegion and SingleChildScrollView are produced.
• packages/mix/CHANGELOG.md: entry under the unreleased 2.2.0-beta.5 section.
• guides/api-composition-guidelines.md: quick-reference lines and a worked widget-modifier example. Both snippets were compiled against the package before being added.
• guides/modifier-config-api-plan.md: removed, now that the plan it described is implemented.

Review Checklist

• Testing: melos run gen:build (no generation drift), melos run analyze (Dart + DCM), and melos run ci all pass. flutter test test/src/modifiers is green at 498 tests, 25 of them new.
• Breaking Changes: None. Additive only; no existing factory or chain-method signature changed.
• Documentation Updates: Dartdoc on the new members, a CHANGELOG entry, and the composition guide.
• Website Updates: Not included. The btwld/mix-docs site and the DartPad examples should keep the existing .modifier(...) form until these APIs land in a published release.

Additional Information (optional)

Pre-existing defect found while testing, deliberately not fixed here. MouseCursorModifier and ScrollViewModifier are absent from _defaultOrder, so reorderModifiers appends them in chaining order and always places them innermost:

scrollView().padding().mouseCursor()  ->  Padding, ScrollView, MouseCursor
mouseCursor().padding().scrollView()  ->  Padding, MouseCursor, ScrollView

In practice .scrollView(...).padding(...) puts the padding outside the scroll view, and the two modifiers' relative order depends on call order. This predates the PR, but it matters more now that both are first-class APIs. Related and also pre-existing: the defaultModifier lerp-fallback map omits MouseCursorModifier, ScrollViewModifier, ClipRectModifier, ShaderMaskModifier, DefaultTextStylerModifier, and BoxModifier, so animating those from absent to present snaps at t = 0.5 instead of lerping. Both are worth a separate follow-up.

The branch name still carries the docs/ prefix from the plan-only branch this work grew out of; the change itself is a feat.