Add a Column Link option to the native Columns block

[Castellon-ACM]

Summary

Closes #295.

Adds a "FrontBlocks - Column Link" panel to the native core/column block's Inspector Controls. Setting a URL makes the whole column clickable on the frontend (optionally in a new tab), without hijacking clicks on a real link, button, or other interactive element already inside it.

• Attributes registered server-side via register_block_type_args for core/column only (frblColumnLinkUrl, frblColumnLinkNewTab).
• Output rewritten at render time via render_block_core/column — the wrapper gets class="frbl-column-link", data-frbl-column-link-url (escaped with esc_url(), which also rejects unsafe schemes like javascript:), role="link", tabindex="0", and data-frbl-column-link-target="_blank" when new-tab is enabled. Nothing is altered client-side in the saved block content.
• Frontend assets (a small stylesheet + a vanilla-JS click/keyboard handler) are only enqueued on pages that actually contain a linked column.
• Keyboard accessible: Enter/Space activates the column when it itself carries focus; a nested control keeps handling its own activation.
• The editor canvas shows a dashed-outline hint on a linked column without making the canvas itself navigate on click.
• Follows the same pattern already used for core/columns (ColumnsSameHeight) and core/button (DownloadButton).

Test plan

• composer lint and composer phpstan clean
• Full PHPUnit suite passing (518 tests) — new ColumnLinkTest.php covers attribute registration, the render_block rewrite (classes/attributes/escaping), the unsafe-scheme rejection, and asset registration/enqueueing
• New tests/js/column-link-frontend.test.js (Node test runner, matching the existing frontend-JS test pattern) covers click delegation (background click navigates, nested link/button clicks are not hijacked), new-tab handling, and keyboard activation
• Live-verified in a real browser against a test page: correct attributes/CSS on the rendered column, a nested link click navigates to its own href (not hijacked), a background click navigates to the configured URL, and pressing Enter while the column is focused navigates the same way

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