chore(api): ship the first PPTX backend release as Beta

[DemchaAV]

Why

The fixed-layout PPTX backend ships its first release in 2.1.0. The geometry-identity design is settled, but the API shape around it — the handler seam, the environment surface, the convenience trio — has not yet seen outside feedback. Left unmarked, all of it would default to the Stable tier and lock its shape until a major release; the stability policy's tool for exactly this situation is @Beta (Experimental), which keeps the right to adjust the contract in a minor while feedback lands.

What

• Package-level @Beta on document.backend.fixed.pptx and …pptx.handlers, plus explicit class-level markers on the five entry types users actually touch — PptxFixedLayoutBackend, PptxFixedLayoutBackendProvider, PptxFragmentRenderHandler, PptxRenderEnvironment, PptxCoordinates — each with an Experimental paragraph in its Javadoc pointing at docs/api-stability.md.
• Method-level @Beta on the DocumentSession PPTX convenience methods (toPptxBytes, writePptx, both buildPptx overloads); the shared Experimental note lives on toPptxBytes, which the other three already reference for the geometry and classpath contract.
• docs/api-stability.md names the PPTX backend as the current Experimental surface and records the one thing that is not subject to change: geometry identity with the PDF backend is a design invariant.
• CHANGELOG Public API entry and a beta marker on the README backend line.

Verification

• Full reactor gate green: clean verify across core, render-pdf, render-docx, render-pptx, templates, testing, root, bundle, qa, coverage — annotations are runtime-retained and the architecture guards (BetaAnnotationDocumentationTest, InternalAnnotationCoverageTest) stay green.
• Core Javadoc gate green after the Javadoc additions (javadoc:javadoc -pl :graph-compose-core).
• No behaviour change: the diff is annotations, Javadoc, and docs only.