feat(engine): PathNode vector-path primitive with native PDF curve operators

[DemchaAV]

What

W1 of the path workstream (smooth charts → design shapes → SVG import): a general-purpose PathNode primitive — the open-path, curve-capable sibling of PolygonNode, mirroring its stack one-to-one:

• DocumentPathSegment (sealed: moveTo / lineTo / cubic cubicTo / close) — normalized unit-box coordinates in the ShapePoint orientation, but carried as raw finite doubles: Bézier control points deliberately may overshoot the box (Catmull-Rom→Bézier conversions need it; ShapePoint clamps to [0,1], which is why it isn't reused).
• PathNode (atomic, fill via non-zero winding and/or stroke) + PathDefinition + PathFragmentPayload + PdfPathFragmentRenderHandler emitting native moveTo/curveTo operators — curves stay perfectly smooth at any zoom, no tessellation.
• Registration only: one line in BuiltInNodeDefinitions, one handler in defaultHandlers() (dispatch is map-first — no chain edits). Zero hot-path changes.

Verification

• 9 new tests: PathNodeTest (MoveTo-first, ≥2 segments, finite coords, overshoot allowed, defensive copy), PathNodeRenderingTest (atomic placement, stable paths, committed snapshot baseline document/path_primitive, valid PDF), PathPrimitiveDemoTest (wave + blob + mixed ribbon vs blank diff, writes target/visual-tests/path/path_primitive.pdf).
• Rendered demo eyeballed: sine wave is perfectly smooth (no facets), filled blob with overshooting controls, mixed line/curve filled ribbon.
• API hygiene checker green; full gate ./mvnw verify -pl . — 1254 tests, 0 failures, BUILD SUCCESS.

Next: W2 — DSL addPath(...) builder for design shapes; W3 — smooth chart lines + curved area fills through this primitive.