fix(ios): resolve computed weights for variable fonts - #11619

User-visible bug

Apps often bundle one variable font file to show regular body text, semibold labels, and medium-weight form fields. On iOS, these can all appear too thin because CSS font-weight leaves the font at its default instance.

For example, SN Pro defaults to weight 200 (ExtraLight):

Authors should be able to use ordinary CSS classes and inherited weights without also writing low-level font-variation-settings for every control.

Fixes #11618.

Change

• Apply the computed CSS weight to the font's supported wght axis, including normal/400 defaults.
• Keep explicit variation settings in control, preserve other axes and font traits, and clamp weights to the font's range.
• Cover labels, text fields, and text views through the shared iOS font resolver. Static fonts retain their existing selection.

Validation

• Passed: core build, 643 unit tests (1 skipped), formatting, and focused native-test typecheck.
• Added: native SN Pro regressions for defaults, classes, inheritance, retained-control updates, explicit-axis removal, and font scaling.
• Device evidence: equivalent downstream tests passed on an iOS 27 simulator. The new upstream automated-device tests have not been run.