feat(charts): smooth lines render as native cubic Beziers via PathNode

[DemchaAV]

What

W3 — the payoff of the path workstream (#168 primitive, #169 DSL): ChartSpec.line().smooth(true) stops tessellating.

• LineChartLayout drops the fixed 8-sub-segment Catmull-Rom sampler entirely: a smooth run (3+ points) compiles into one stroked PathNode with Catmull-Rom-derived Bézier control points (c1 = p1 + (p2−p0)/6, c2 = p2 − (p3−p1)/6 — the exact continuous curve the sampler approximated). Native PDF curveTo operators, zero facets at any zoom, and fewer content-stream ops than the old 8×N segments.
• Smooth area fills close the very same curve down to the baseline — fill and stroke edges coincide instead of a tessellated polygon hiding under a sampled stroke.
• Two-point runs stay straight segments; non-smooth charts untouched — all committed chart snapshot baselines pass unchanged.

Verification

• Resolver tests rewritten/added: one-native-Bézier-run (segment count + cubic types + stroke/fill split), curved-area shape (moveTo + 2 cubics + 2 baseline edges + close), two-point fallback, one-point smooth+area edge case still green.
• ChartLayoutSnapshotTest (committed baselines) green without re-baselining.
• Regenerated chart-showcase.pdf + feature-catalog.pdf previews; the smooth-area page eyeballed — perfectly smooth, fill hugs the stroke.
• Full gate: ./mvnw verify -pl . — see commit; BUILD SUCCESS.