Create a page and its Main Subject in one step

[malberts]

Fixes #1355

A Subject could only be created from a page that already existed: create the page, name it, open Create subject,
pick the Schema, name the Subject again. Nothing said the page was a prerequisite, and users who think in entities
expected Create subject to create the thing itself.

What it does

Schema-first creation, with the page created on the way:

• Special:CreateSubject opens the Subject creator with a Page field: matching existing pages plus "Use X as a new
  page". A subpage pins the Schema (Special:CreateSubject/Person).
• A Create Person call to action below the description of Schema:Person (named after each Schema), leading
  there with the Schema pinned.
• Create subject in the sidebar links there from every page. The page tool that adds a Subject to the page being
  viewed is now called Create subject here, and the Data tab's button Create subject, so every entry uses
  one verb.

Picking a page that exists adds the Subject to it: as its Main Subject when it has none, otherwise alongside the one
it has, which the dialog names. A new page is created empty on save and the Subject becomes its Main Subject; the
browser then lands on that page.

Decisions worth challenging

The label is not copied from the page title. A Main Subject without a label already displays as its page name
(ADR 31), and a stored copy would be exactly the default that ADR removed: it cannot be told from a chosen label, so
a page move would stop renaming the Subject. The page title is the required input; the label stays optional.

Two writes, not one endpoint. The page is created through MediaWiki's own API and the Subject through the
existing endpoint, as the move dialog does. Core enforces createpage, nothing is written until save, and the page id
is kept so a retry after a failed Subject write does not create the page twice. The cost is an empty page when that
retry never comes, and two recent-changes entries. A NeoWiki endpoint writing both slots in one revision is the
follow-up if that bites.

The page picker rather than a title field. A title collision becomes a visible choice between the existing page
and a new one, instead of a silent merge or an error.

A link rather than an in-place dialog on Schema pages. One mount point, and a URL that later entry points (red
links, in-flow creation) can reuse.

A special page rather than a sidebar link that opens the dialog in place. The frontend bundle (Vue, Codex,
Pinia and NeoWiki's own code) is loaded only on content, Schema, Layout and Mapping pages and on NeoWiki's special
pages. Anywhere else the link would first have to load all of that before a dialog could open, so a page that loads
it is the plain, reliable path. The URL is also what makes the entry point placeable: [[Special:CreateSubject/Person]]
works in any page, template or MediaWiki:Sidebar without JavaScript, and it is what the red-link affordance for
missing relation targets (#1120) can point at. Opening the dialog in
place where the bundle is already loaded is a possible follow-up, not a change of approach.

The special page is restricted to edit and refuses users without createpage. A special page names one
restriction; edit is the one the Subject write needs and the one anonymous users lack on hosted wikis, so the page is
hidden from them. The sidebar link and the Schema page button require the same pair of rights.

Considered, omitted

• Prefilling the page title from a URL parameter.
• Edit notices for the chosen page.
• Locking a pinned Schema; the back button still reaches the picker.

Screenshots (UI, Playwright)

Sidebar on a content page. The page tool now reads "Create subject here"; "Create subject" is the new link to the special page.
Schema page. "Create Person" below the description, with the pencil alone in the corner.	[SCREENSHOT: schema-page-create-button.png]
Special:CreateSubject/Person. The dialog on the Person step, with a new page chosen in the Page field.
The created page. Its Main Subject rendered as the infobox under the page name.
An existing page chosen. The notice naming the page's Main Subject; the new Subject is added alongside it.

Testing

Vitest covers the creator's page-first mode (page required, new page created then written as Main Subject, existing
page with and without a Main Subject, retry without a second creation, the taken-title error), the special page's
frontend, the Schema page button and its permission hint, and the shared page creation. PHPUnit covers the special
page (mount point, subpage normalization, both refusals) and the sidebar link and its gate. The save path's guards
(the page and tier decided when the save started, no second submit while one is in flight, stale reads of a page the
user moved off) each have a test that fails without them.

_{AI-authored — Claude Code, Fable 5.1 (high); one-line ask from @malberts, design proposals approved without changes, one mid-task instruction on process; diff reviewed by two Opus subagent passes (code review, mutation-checked test review) with their findings applied, not yet human-reviewed; vitest, phpcs, phpstan and the full PHPUnit suite pass locally, Playwright check on the dev stack with screenshots, CI pending.}