docs(readme): make the hero the banner the release advertises

[DemchaAV]

Why

The image at the top of README was a page of the module-first deck, and the caption beside
it linked a third document again. The Maven banner is the one written to be seen first — it
names the coordinate, the version and the outputs, and the release re-renders it on every
version bump — and nothing on the front page pointed at it.

2.2 is the right-to-left line, and the banner said nothing about it.

What changed

• The hero is page one of the Maven banner.ReadmeHeroRenderer replaces
  ReadmeBannerV2Renderer and writes the same assets/readme/repository_showcase_render.png,
  so the release keeps its staging list and the version stamped into the image still arrives
  from the version bump. One line of the script names the new class.
• The banner gained a fifth page. Amiri, David Libre, the two Noto faces and Gothic A1
  each set a word, and where that word sits in its card is the direction it declared — Hebrew
  and Arabic to the right, the rest to the left. Three cards below say what the engine does
  with them: resolve the line through UAX Backport to Java 17 #9, shape Arabic before measuring it, hand each
  backend what its own text engine needs.
• The README caption links maven-banner.pdf and the example that renders it. The gallery
  row and the showcase blurb stop calling the banner a single slide — it has been four pages
  since before this change.
• EngineDeckV2Example.renderBannerImage is removed: the hero was its only caller, and
  the class comment that said the switch had not happened yet was written before it did.
• A guard on the step that renders the hero. The class name and the output path are
  strings the compiler never reads: the script hands the class to exec:java as text, and one
  part of it writes the file while another stages it. Each half is now read out of the script
  rather than restated here, so a rename or a dropped staging line fails at test time instead
  of mid-cut. Driven from the script's own text in five broken states.
• Two sentences that stopped being true in this release: the RTL examples entry still said
  a table cell has no direction, and the drift test still said nothing moved the version its
  previews are compared at.

Tests

./mvnw -B -ntp clean verify → BUILD SUCCESS.
./mvnw -B -ntp -f examples/pom.xml test → BUILD SUCCESS, 75 tests.

Two guards moved with the fifth page: the deck's slide count and its committed preview. The
new page is native — the deck's only rasterised element is still the checkmark in the badge.
The hero renders at 2666×1500, the size of the file it replaces, and carries 2.1.1 rather
than the working tree's version.

Note

The deck goes from 41 KB to 1.3 MB. All of it is fonts: PowerPoint is handed each of the five
families whole rather than subset, and Gothic A1 alone is 688 KB compressed. The PDF grew by
16 KB, because a PDF embeds the glyphs it draws.

That is the page working rather than the page costing. A deck that sets Hebrew and Arabic and
carries no fonts is a deck that renders them on whatever the reader happens to have installed —
which is the problem the bundled families exist to end, and what anybody shipping a right-to-left
document pays too. Subsetting the embedded copy would cut it, and that is a backend change with
its own tests, not a reason to take a script off the page.