feat(api): PPTX convenience trio on DocumentSession

[DemchaAV]

Why

Rendering a deck required importing the backend and calling session.render(new PptxFixedLayoutBackend()) by hand — the session's chrome (metadata, watermark, headers/footers) silently did not apply on that path, and there was no PPTX equivalent of the toPdfBytes/writePdf/buildPdf convenience surface.

What

• DocumentSession.toPptxBytes(), writePptx(OutputStream), buildPptx(), and buildPptx(Path) mirror the PDF trio's contracts: caller-owned streams stay open, the no-arg build uses the builder's default output file, and the session's chrome flows into the deck exactly as it does into the PDF.
• The backend resolves through the format-keyed provider (BackendProviders.fixedLayout("pptx")), so core gains no PPTX dependency; without graph-compose-render-pptx on the classpath the call fails with the existing MissingBackendException naming the artifact to add.
• Internally the facade's PDF paths and the new PPTX paths share one format-generic pipeline (renderBytes/writeFixedLayout/buildFixedLayout), byte-for-byte faithful to the previous PDF behaviour — log keys, null checks, and exception flow unchanged apart from the format placeholder.
• Docs follow the surface: capability matrix row, docs/capabilities.md output table, lifecycle diagram, the empty-document and default-output-file diagnostics, and the MissingBackendException examples now name both formats.

Verification

• 7 new session-level tests: chrome flow-through on a two-page deck (metadata in OPC properties, footer tokens restarting per page), slide-count identity with the resolved layout graph, caller-owned-stream contract, explicit-path and default-file builds, and the failure modes (no default file, empty session, null stream passing through unwrapped, closed session).
• Core's backend-free MissingBackendContractTest already pins the "pptx" missing-format diagnostics.
• Full reactor gate green: clean verify across core, render-pdf, render-docx, render-pptx, templates, testing, root, bundle, qa, coverage.