fix: preserve irregularly spaced graph points

[huytdps13400]

Summary

• sample graph data using each point's timestamp-derived X coordinate
• preserve irregularly spaced intermediate values that the index-based sampler skipped
• keep the existing two-pixel density cap and always retain boundary points

Fixes #103.

Root cause

The path sampler selected a data index as if points were uniformly distributed across the X range, then required that point's actual timestamp-derived X coordinate to match the sampled pixel. For irregular time series, those two positions diverge, so valid intermediate values were dropped from the Skia path.

The sampler now walks the already-filtered, chronologically ordered data and applies the same density limit to each point's real X coordinate. This keeps path command counts bounded while representing irregular timestamps correctly.

Verification

Using the repository Node.js version (22.20.0):

• focused regression observed RED before the fix: expected 3 cubic commands, received 2
• yarn test --runInBand — 2 suites, 4 tests passed
• yarn typecheck
• yarn lint — 0 errors; one pre-existing no-shadow warning in src/AnimatedLineGraph.tsx
• yarn prepare
• CI=1 yarn workspace react-native-graph-example expo export --platform web --output-dir /tmp/graph-103-web-dist
• git diff --check

Scope

No public API or dependency changes. Static and animated graphs continue to create paths only when their input/layout changes, not per animation frame.