[6.x] Fieldset sections

[jackmcdade]

Adds support for sections inside fieldsets and makes blueprint fieldset imports section-aware.

• Fieldsets can now be authored with section structure (while flat fieldsets remain supported).
• Blueprint imports now support section_behavior with preserve (default) or flatten.
• CP import UI now indicates when a linked fieldset has sections and exposes section behavior controls only when relevant.

Notes

• Flat fieldsets are kept flat on save (single default “Fields” section is collapsed back to fields).
• Preserved sections will split the current section out wherever it is. If there are fields after it, they will be in their own section below the imported fieldset section.
• Includes backend + CP updates and test coverage for parsing, rendering, import behavior, and persistence.

References statamic/ideas#988

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Note

Medium Risk
Changes how fieldsets are represented, edited, and serialized (new sections structure with normalization), plus modifies publish rendering for imported fieldsets; regressions could affect CP editing and blueprint/fieldset import layouts.

Overview
Adds first-class sections inside fieldsets: the CP fieldset editor now uses the Sections UI (with drag/drop across sections), and the backend Fieldset model normalizes storage to either flat fields or non-empty sections (never both).

Makes fieldset imports section-aware: import settings gain a section_behavior (preserve default vs flatten), the CP import UI shows badges/controls only when the linked fieldset has sections, and Tab::toPublishArray() can expand sectioned imports into publish sections (or flatten them in place).

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