Add FCollapsible.axis for horizontal collapsing

[Pante]

Describe the changes

Closes #974 (the axis half of the request).

• Add FCollapsible.axis (defaults to Axis.vertical). Axis.horizontal collapses the child along its width instead of its height; the render box and clipper now branch on the axis.
• Non-breaking: the Axis.vertical default is byte-identical to the previous hardcoded behavior, so FAccordionItem and FSidebarItem are unaffected.
• Add a half expanded horizontal golden and rename the vertical half expanded golden to half-expanded-vertical (the fully expanded/fully collapsed cases render identically on both axes, so they stay unqualified).
• Add a behavioral test asserting per-axis sizing and relayout on axis change.
• Surface axis in the usage snippet and add a CHANGELOG.md entry.
• Note: the implicit-animation part of FCollapsible enhancement #974 is intentionally not included, per the maintainer comment on the issue (collapsible stays a primitive so higher-level widgets can drive their own animations).
• Note: only the macOS goldens are regenerated here; the Windows half-expanded-{vertical,horizontal}.png baselines will be produced by the golden CI workflow when it resumes.

Checklist

• I have read the CONTRIBUTING.md.
• I have included the relevant unit/golden tests.
• I have included the relevant samples.
• I have updated the documentation accordingly.
• I have added the required flutters_hook for widget controllers.
• I have updated the CHANGELOG.md if necessary.

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