Support native field category assignments and preserve partial updates

[Llewellynvdm]

Custom field category selection was absent from create/update schemas, and title-only API PATCH could silently erase the separate native category relation. This change adds a component-owned typed assigned_cat_ids extension for all six field contexts and both API/CLI transports, preserves omitted assignments, and verifies native All/None/category membership semantics. The subform-only None override uses explicitly approved input.

Original migration snapshots remain intact. Regenerated seeds preserve administrator-customized bindings and schemas. Missing assignment data during API update preflight fails before writing.

Corresponds to joomengine/joomla-mcp#56. Paired TypeScript fix: joomengine/joomla-mcp#58.

Validation: all 35 local CI suites passed; the new category suite passes 653 checks; source ZIP package validation passes 1,879 checks; generated seeds reproduce exactly. The existing installed field-defaults suite now creates fields with a real category through API/CLI, checks title-only PATCH preservation and explicit category/All/None/subform selections. Installed Joomla execution is covered by the PR workflows.

[Llewellynvdm]

All workflows are green on final head 944c788:

• PHP 8.3/8.4 contracts and migration evidence: https://github.com/joomengine/mcp_component/actions/runs/38048784331
• Installed Joomla, PHP 8.3/8.4 with MySQL/PostgreSQL: https://github.com/joomengine/mcp_component/actions/runs/38048784299
• Installed JCB golden image: https://github.com/joomengine/mcp_component/actions/runs/38048784316

The installed logs confirm API and CLI field creation with a real category, title-only update preservation of the separate relation, explicit category/All/None selections and the approved subform-only [-1] result.