Add disabled attribute support for form controls

[SpinyOwl]

What changed

• add boolean disabled attribute support for button, input, and textarea
• expose NodeBuilder.disabled() and live control disabled() state based on attribute presence
• suppress mouse focus/click/action, keyboard activation/editing, and char input for disabled controls
• add CSS :disabled pseudo-class support
• add focused regression tests for state, selector matching, and system event suppression

Semantics

• disabled follows boolean attribute presence semantics: disabled="false" is still disabled until the attribute is removed
• unsupported generic elements do not become disabled merely by carrying a disabled attribute
• disabled button-like controls do not emit ActionEvent

Validation

• GitHub Actions will run against this PR; local checkout execution was unavailable in the current environment.

[SpinyOwl]

CI validation note: the final head compiles production and test sources successfully and runs 603 core tests. The Gradle clean build is blocked by exactly one existing failure: AbsoluteContainingBlockLayoutTest.layout_whenAbsoluteAutoHeightUsesTopAndBottom_sizesBeforeNestedLayout() with NullPointerException at line 81. The same failure reproduced across earlier PR runs, and this PR does not modify layout code. No disabled-related test failures were reported. CodeQL's Gradle build is consequently blocked by the same test before analysis.

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