feat(split): migrate si-split to OnPush

[spliffone]

BREAKING CHANGE: si-split and si-split-part inputs were migrated to signal inputs

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Documentation.
Examples.
Dashboards Demo.
Playwright report.

Coverage Reports:

• charts-ng
• dashboards-ng
• element-ng
• element-translate-ng
• maps-ng
• native-charts-ng

[gemini-code-assist]

Code Review

This pull request refactors the split component and its parts to migrate from traditional decorators to Angular's modern signal-based APIs, including input(), contentChildren(), and linkedSignal(), alongside updating templates and unit tests to use signal getters and modern async test stabilization. The review feedback highlights two critical improvements: refactoring imperative effect() blocks in SiSplitPartComponent to declarative computed and linkedSignal properties to align with the repository style guide, and declaring grid template signals as read-only computed class properties in SiSplitComponent instead of dynamically reassigning them.

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