feat(pptx): complete the fragment matrix — images, barcodes, paths, transforms

[DemchaAV]

Why

The fixed PPTX backend rendered text, tables, and basic vector shapes, but images, barcodes, polygons, free paths, gradients, and transform markers still threw UnsupportedNodeCapabilityException — the last gaps between "renders some documents" and "renders any resolved LayoutGraph".

What changed

• Drawing-surface indirection: handlers now draw on PptxRenderEnvironment.surface(page) — the innermost open transform group or the page's slide. Transform begin/end markers become containment: the begin marker opens an XSLFGroupShape whose interior anchor is the fragment box while the exterior anchor scales about the centre and carries the clockwise rotation — PPTX's equivalent of the PDF backend's ambient graphics-state matrix, and it applies to every fragment kind between the markers because they all draw through the surface.
• Images (PptxImageFragmentRenderHandler): the PDF fit modes — STRETCH fills the box, CONTAIN letter-boxes the centered bitmap, COVER crops through the picture's source rectangle (DrawingML's native crop). Source dimensions decode from the real bitmap with a per-fingerprint cache: POI's header sniffing mis-reports some images and would break the aspect math the PDF backend computes from decoded pixels.
• Barcodes (PptxBarcodeFragmentRenderHandler): a byte-mirror of the PDF handler's ZXing raster — same writers, hints, oversampling, and pixel mapping — so both formats carry the same bitmap.
• Polygons and free paths as freeform geometry (PptxInlineGeometry reuse), with the engine's y-up normalization flipped into slide space.
• Gradients (PptxGradientFill): linear gradients stamp native gradFill (engine's counter-clockwise angle converts to DrawingML's clockwise sixtieths — 90° becomes 270°); gradient strokes use ln/gradFill natively, clearing the noFill POI writes for a null line color (PowerPoint would otherwise honour noFill and drop the stroke). Radial centres and explicit linear axes approximate with a one-time note. The rectangle shape handler now stamps real gradients instead of the primary-color fallback.
• Clip markers degrade to a one-time warning — DrawingML has no graphics-state clipping; exact clips belong to the PDF backend or raster-slide mode. Anchor and bookmark markers record their destinations for the navigation pass. With these, every payload the layout compiler emits has a handler; the unsupported-payload diagnostic is pinned with a synthetic unknown payload through the public render path.

Verification

./mvnw -B -ntp clean verify (full 10-module reactor gate) → BUILD SUCCESS; render-pptx suite 34 → 39 tests.

• PptxVectorFragmentsTest — fit-mode anchors and the COVER crop against the captured graph; barcode picture on the fragment box; synthetic polygon and gradient-path freeforms asserting the raw CT: stop count, the 90°→270°×60000 angle conversion, ln/gradFill present with no leftover noFill and the exact EMU width; transform groups: rotation, interior anchor = fragment box, exterior scaled about the unchanged centre.
• PptxVectorParityDemoTest — the reviewable pair + PNGs under render-pptx/target/visual-tests/pptx-parity/vector-fragments/: images and the QR are visually identical; the rotated badge shows the documented clip degradation (PDF clips the layer to the container outline, PPTX renders it unclipped with the warning).

Notes

• Numeric dash arrays still map to the dashed preset (documented since the shapes slice).
• Group nesting relies on the compiler's same-page begin/end balance; an unmatched end now fails with a clear IllegalStateException instead of an empty-stack error.

Lane: canonical (document.backend.fixed.pptx) — backend surface; engine, layout, and the PDF paint path untouched.