test(qa): PPTX boundary guards, flagship deck example, CI install step

[DemchaAV]

Why

The PPTX backend's module boundaries had no bytecode-level guards (nothing stopped POI from leaking into the engine or a template, or the PDF backend from growing a PPTX dependency), the ShapeOutline exhaustiveness guard only proved the PDF dispatch chains, and the flagship Engine Deck rendered only to PDF — so the new backend had no dev-facing example and the examples CI job could not have resolved it anyway.

What

• qa guards. graph-compose-render-pptx joins the qa test classpath. CrossModuleBoundaryArchTest gains three rules: POI stays a render-backend implementation detail (only the PPTX backend packages — and, for the day render-docx joins this classpath, the DOCX semantic backend — may touch it), the render backends stay template-agnostic, and the PDF backend must not depend on the PPTX artifact (the reverse dependency is by design: the PPTX backend reuses the PDF measurement stack). ShapeOutlineRenderCoverageTest now renders every permit through all three surfaces on both fixed-layout backends; PptxInlineGeometry.drawOutline gains the terminal IllegalStateException that makes the inline PPTX chain discriminating — without it a new outline kind would silently render nothing and the guard would still pass.
• Flagship example. EngineDeckPptxExample renders the exact Engine Deck composition (shared package-private compose(...) seam — banner, pipeline, two benchmark pages with tables and native charts) as a geometry-identical .pptx via buildPptx(); registered in GenerateAllExamples, examples README row extended.
• CI. The examples job installs graph-compose-render-pptx before generating, mirroring the render-docx step; the test-job reactor comment now describes render-pptx as the fixed-layout backend.
• README. Backend descriptions present graph-compose-render-pptx as the fixed-layout, geometry-identical deck backend (clipped regions land as pixel-exact pictures) with its own quick-start dependency row, replacing the stale "semantic export" framing.

Verification

• New guards green: CrossModuleBoundaryArchTest 6/6, ShapeOutlineRenderCoverageTest 4/4 (every permit × three surfaces × two backends).
• EngineDeckPptxExample generated locally: 4 slides matching the 4 PDF pages; rasterized side-by-side previews of the banner and benchmark pages are visually identical.
• Full reactor gate green: clean verify across core, render-pdf, render-docx, render-pptx, templates, testing, root, bundle, qa, coverage.