126c191252fda4522a37db35b2eadc441243d360  Add parsing support for flex-wrap: balance

Classified as not security-relevant.
Triage note: Implements the css-flexbox-2 'balance' flex-wrap value (new CSSFlexWrapValue/Style::FlexWrap types, parser grammar, cssFlexWrapBalanceEnabled default false). Pure new-feature work.
This commit was flagged by the signal scorer and reviewed by AI (not a human). It is listed only so the triage is auditable; there is no root-cause analysis because this is not a vulnerability fix.

Structural signals: smartptr_member_added (+6.47), security_assert (+3.0), null_guard (+1.2), msg_noise (-2.5), area:Source/WebCore/css (+2.0), large_churn (-2.0)