refactor(mix): consolidate example widget modifiers

[leoafarias]

Related issue

Companion guide update: btwld/website#57

Description

The Glide Select and Thought Line examples repeat .wrap() calls where one chained modifier configuration expresses the same style. Consolidate those chains and the Mix skill's opacity/rotation example without changing animation states, values, or wrapper order.

Changes

• Chain modifiers within one .wrap() in the two showcase styles and skill example.
• Add regression coverage for both boolean animation states, rotation composition, and rendered default/custom wrapper order.
• Preserve separate state-specific modifiers.

Validation

• melos run gen:build passed.
• melos run ci passed (4,914 tests across the configured suites).
• 30 focused modifier tests passed, including the new regression cases.
• Schema inventories and melos run analyze:dart passed; changed files have no diagnostics.
• Formatting and git diff --check passed.
• melos run analyze reaches DCM but fails on four pre-existing issues in unchanged style_animation_builder.dart and decoration_style_mixin.dart (one warning, three style issues). Both files match the base commit.

Review Checklist

• Testing: Unit/widget coverage and workspace suites run.
• Breaking Changes: None; this changes examples only.
• Documentation Updates: Mix skill example updated.
• Website Updates: Companion draft PR linked above.

GitHub CI: all 12 checks passed for c1de4c9b4d6e2d5fc4796ad0f289984c8a4bf924, including the full workspace suite, published examples, formatting, protocol checks, and CodeQL.

[tilucasoli]

LGTM