Show a Subject's Schema as one badge wherever it is named

[alistair3149]

For #1328

A Subject's Schema was drawn four ways: a permission-gated link in the editor header that opened the
Schema editor rather than navigating, subtle text beside a navigator row, a subtitle link in the
infobox, and another beside a statement count in the Subjects manager. Nothing about any of them said
"this string is a Schema" — the register they share is also the register of statement counts, subject
ids and the tree's group captions.

SchemaNameDisplay renders the bare name as a badge and owns the rules each surface used to decide
for itself: where the link goes, whether it opens a tab of its own, and when the name is withheld
because it would only repeat the name beside it.

Decisions worth challenging

A badge, deliberately not a CdxInfoChip. A chip is a bordered pill at @font-size-small, which
is the size of a navigator row's own text; thirteen of them down the demo wiki's tree read as a wall.
The badge is a step smaller, filled rather than outlined, and on @background-color-neutral rather
than -subtle, which would all but vanish on a hovered row and clash on a selected one. Chip and
badge stay distinct shapes for distinct ideas — the chip means Property Type on the Schema page, and
this PR leaves it alone.

The tree gets a slot rather than a stylesheet. NeoTree gains a secondary slot, rendered
inside the element that held the plain text, so the tree learns nothing about Schemas and one
component still owns the badge. The row's DOM is unchanged, which matters:
SubjectTree.spec.ts:809 asserts a row's children and text exactly, to catch a re-introduced glyph,
and it still does.

Always a link, and the withholding rule moves in. Reading a Schema is not an edit right, so the
badge no longer hides behind neowiki-schema-edit. schemaNameToShow is shared by the badge and by
the tree's own secondaryLabel, so a Subject displayed under its Schema name (ADR 31) shows it once
— the infobox and the Subjects manager printed it twice before.

Not here: the editor's own header. It keeps Schema: Company for now. The follow-up removes that
subtitle wholesale, and changing its click target here would quietly retire the only in-dialog route
to the Schema editor — a call that belongs with the PR redesigning that header. So this PR leaves the
editor header inconsistent with the tree below it, on purpose and briefly.

Also here: neowiki-infobox-type is deleted, being declared and defined but referenced by no code;
DataExportButton's copy of the visually-hidden rules moves to a shared mixin.

Testing

1689 tests pass, up from 1673 — sixteen new. The new ones were mutation-verified: inverting the withholding rule,
making new-tab stop opening one, linking the tree's badge, removing the badge from the tree, and
dropping displayName at the infobox and both manager call sites each turn a test red by name. The
last three of those found real gaps — the first cut of this PR had no test that would have noticed a
call site forgetting to pass the name it displays.

Manual Browser Check

On a wiki with the demo data, logged in:

1. Open ACME Inc. Under the infobox title, Company is a small filled badge, not blue link text.
   It links to Schema:Company in the same tab. Hovering underlines it.
2. Open a page whose Main Subject has no label of its own — its title already is its Schema name.
   The badge is absent rather than repeating the title. (Special:Subjects on such a page shows the
   same: the row named after its Schema carries no badge, one named after its page does.)
3. Click the infobox pencil. Every navigator row carries the same badge. Rows stay 32px; the badge
   does not push them taller. The badge is not a link here — the row is the click target.
4. Select a row. The badge still separates from the selected row's blue-tinted background.
5. Switch the skin to dark. The badge follows: dark fill, light text, same contrast relationship.
6. With a screen reader, the badge announces "Schema: Company" — the noun is present once, not twice.
7. The dialog header still reads Schema: Company as a link that opens the Schema editor. That is
   deliberate for this PR; the follow-up removes it.

_{AI-authored — Claude Code, Opus 5 (1M context); design chosen by @alistair3149 from options put to them in a design artifact, including the badge-not-chip call and the decision to unify behaviour as well as appearance; diff not yet human-reviewed; verified: 1689 vitest tests, lint and build green locally, six mutations confirmed red, and the badge checked in a browser against a dev wiki in light and dark, in the infobox, the Subjects manager and the editor's navigator.}

[alistair3149]