Add Related-entities title-bar navigation with open PHP/JS filter surface

[AllTerrainDeveloper]

What

A "Related" title-bar button (network icon, right of the ⋯ menu) on any window whose content identity carries navigation targets. Clicking it opens a grouped dropdown; picking an item opens that entity as its own desktop window.

Built-ins cover posts and pages:

• Comments → edit-comments.php?p={id}, with the approved + pending count the opened screen actually lists
• Categories / Tags → one item per assigned term → term.php?taxonomy=…&tag_ID=… (grouped per taxonomy, budgeted at 32 across taxonomies)
• Media → one item per associated attachment (featured, attached, wp-image-{id} embeds, deduped, capped at 20) → upload.php?item={id}

Developer surface (the point of the PR)

Entity-agnostic, both ends open — e.g. a payments plugin can show "Order #77" and its line items on a payment-detail window with two filters:

• PHP — desktop_mode_window_related_entities( $related, $identity, $screen ): runs in real admin context, after desktop_mode_window_content_identity and only when an identity resolved, so plugin-injected identities for custom screens get the pass too. Malformed entries are sanitized server-side, mirroring the JS engine's validation exactly — one bad item can never cost a window its whole identity.
• JS — desktop-mode.related-entities.items (( items, { windowId, content } )): one resolver feeds both the button's visibility and the menu build, so they can't disagree. Filters receive shallow copies — the documented items.push(…); return items idiom can't corrupt the stored identity.

Items ride the existing desktop-mode-content-identity bridge payload as WindowContentRef.related (engine cap 64; built-ins can never reach it). Built-ins attach only while the filtered identity still refers to the detected post — rewriting a gated post's identity leaks nothing.

Supporting changes

• edit-comments.php?p=N now announces a per-post identity rooted at the post → the comments window ties to its post window.
• IDENTITY_PARAMS gains tag_ID + item so term/media deep links get their own windows. Generic id deliberately stays transient (plugin row actions must keep navigating lists in place). One-time migration note: saved geometry for term-edit / media-detail windows resets once, since their ids change.
• Content Graph drops its own term-…/comment-…/media-… window-id suffixes — deriveWindowId now differentiates these itself, and the suffixes forked ids away from every other open path (duplicate windows for the same entity).
• Menu panel reuses the ⋯ panel class (drag-tracker exclusion + positioning — a pointerdown on a menu item must not start a window drag), closes cleanly on repaint/content change, suppresses the post-pick double-click that would otherwise maximize the window, and supports full keyboard navigation (arrows / Home / End / Enter / Escape, focus returned to the trigger).
• The Related menu deliberately does not consult native URL remaps: it exists for filtered deep links, which an id-based native open would drop.

Docs

hooks-reference.md, javascript-reference.md, bridge-protocol.md updated in the same change; new recipe docs/examples/related-entities.md (PHP CPT recipe + JS filter recipe), indexed in examples/README.md.

Testing

• PHPUnit: new tests/phpunit/tests/relatedEntities.php (20 tests — builders, caps, sanitizer edge cases incl. whitespace/'0', filter ordering, identity-rewrite guard, pending-comment gating) — full suite green (1234).
• Vitest: new tests/vitest/related-entities.test.ts (resolver, filter contracts, menu lifecycle, keyboard nav, repaint/stale-panel) + engine related normalization + deriveWindowId regressions — full suite green (2077).
• A high-effort multi-agent code review ran over the diff; all confirmed findings were fixed or explicitly documented (the id identity-param idea was rejected for regressing row-action navigation).
• Manually verified on the local dev stack: button visibility follows navigation, all three groups open their targets, ties draw to the opened windows.

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