docs(readme): show the PPTX backend, fix the architecture diagram, drop the dead star chart (#456)

* docs(readme): put the PPTX backend in the badges and the architecture diagram

The badge row credited PDFBox but nothing for the office formats, so the
stack the release is about was invisible. Apache POI 5.5 now sits beside
it; it backs both PPTX and DOCX.

The architecture diagram was wrong in two directions. It routed
DocxSemanticBackend through the ServiceLoader — render-docx publishes no
service file at all, and the exporter is named directly by the caller —
and it left PptxFixedLayoutBackend out entirely, though that is the
backend the seam actually discovers alongside PDF.

DOCX now branches off the node tree, which is what SemanticBackend
consumes: DocumentGraph is the semantic authoring tree, so the exporter
never sees a layout pass. The prose above the diagram said every backend
registers through the seam and now separates the two paths.

* docs(readme): drop the star-history chart

The embed has been rendering as broken alt text. It is not a wrong URL —
api.star-history.com answers 500 for this repo and for its own, with
"timeout of 10000ms exceeded" from its GitHub call, so the outage is on
their side and there is nothing here to fix.

The usual alternative is no better: starchart.cc answers 200 but the SVG
it returns carries the text "failed to talk with github api: Requires
authentication ... 401", which would paint a red error where the chart
used to be. A section that can only render broken is worse than no
section.

Author: DemchaAV

README.md

@@ -14,7 +14,8 @@
   <a href="https://github.com/DemchaAV/GraphCompose/releases/latest"><img src="https://img.shields.io/github/v/release/DemchaAV/GraphCompose?style=for-the-badge&label=Release" alt="Latest release"/></a>
   <a href="https://central.sonatype.com/artifact/io.github.demchaav/graph-compose"><img src="https://img.shields.io/maven-central/v/io.github.demchaav/graph-compose?style=for-the-badge&label=Maven%20Central" alt="Maven Central"/></a>
   <img src="https://img.shields.io/badge/Java-17%2B-orange?style=for-the-badge&logo=openjdk" alt="Java 17+"/>
-  <img src="https://img.shields.io/badge/PDFBox-3.0-red?style=for-the-badge" alt="PDFBox 3.0"/>
+  <img src="https://img.shields.io/badge/PDFBox-3.0-red?style=for-the-badge" alt="PDFBox 3.0 — PDF backend"/>
+  <img src="https://img.shields.io/badge/Apache%20POI-5.5-blueviolet?style=for-the-badge" alt="Apache POI 5.5 — PPTX and DOCX backends"/>
   <img src="https://img.shields.io/badge/License-MIT-blue?style=for-the-badge" alt="MIT License"/>
 </p>
 
@@ -291,7 +292,7 @@ flow.addAligned(HorizontalAlign.RIGHT, anyFixedNode);
 
 ## Architecture
 
-GraphCompose splits into a **public canonical surface** you author against (`com.demcha.compose.document.*`) and an **internal shared engine foundation** (`com.demcha.compose.engine.*`, marked `@Internal`) that resolves geometry, pagination, and rendering behind it. Since 2.0 that boundary is also a **packaging** boundary: the surface and engine ship in `graph-compose-core`, and each render backend is a separate module that registers through a `ServiceLoader` seam. You author intent; the engine resolves the rest.
+GraphCompose splits into a **public canonical surface** you author against (`com.demcha.compose.document.*`) and an **internal shared engine foundation** (`com.demcha.compose.engine.*`, marked `@Internal`) that resolves geometry, pagination, and rendering behind it. Since 2.0 that boundary is also a **packaging** boundary: the surface and engine ship in `graph-compose-core`, and each render backend is a separate module. The **fixed-layout** backends (PDF, PPTX) register through a `ServiceLoader` seam and consume the same resolved `LayoutGraph`, which is why a deck matches the PDF geometrically. The **semantic** DOCX exporter registers nothing — you name it directly, and it walks the node tree without a layout pass. You author intent; the engine resolves the rest.
 
 ```mermaid
 flowchart LR
@@ -300,7 +301,8 @@ flowchart LR
     C --> D["Engine foundation @Internal<br/>measure → paginate → place"]
     D --> E{ServiceLoader}
     E -->|render-pdf| F["PdfFixedLayoutBackend<br/>PDFBox"]
-    E -->|render-docx| G["DocxSemanticBackend<br/>POI"]
+    E -->|render-pptx| G["PptxFixedLayoutBackend<br/>POI · same LayoutGraph as the PDF"]
+    B -.->|render-docx · named directly| I["DocxSemanticBackend<br/>POI · no layout pass"]
     D -.->|layoutSnapshot| H["Deterministic snapshot<br/>(regression tests)"]
 ```
 
@@ -348,13 +350,3 @@ See [CONTRIBUTING](./CONTRIBUTING.md) for the branch-routing table and the full
 ## License
 
 MIT &mdash; see [`LICENSE`](./LICENSE).
-
-## Star History
-
-<a href="https://www.star-history.com/?repos=DemchaAV%2FGraphCompose&type=date&legend=top-left">
- <picture>
-   <source media="(prefers-color-scheme: dark)" srcset="https://api.star-history.com/chart?repos=DemchaAV/GraphCompose&type=date&theme=dark&legend=top-left" />
-   <source media="(prefers-color-scheme: light)" srcset="https://api.star-history.com/chart?repos=DemchaAV/GraphCompose&type=date&legend=top-left" />
-   <img alt="Star History Chart" src="https://api.star-history.com/chart?repos=DemchaAV/GraphCompose&type=date&legend=top-left" />
- </picture>
-</a>