Fix ECharts/Chart.js rendering bugs found in MCP bug-bash

[Chenglong-MS]

Summary

Fixes four rendering bugs surfaced while exercising flint-chart-mcp as an external user in a bug-bash, each with a regression test.

#	Backend / template	Bug	Fix
1	ECharts line / area / streamgraph	The time axis parser only accepts ISO-8601 strings, so human-readable dates like "Jan 1 2000" (e.g. vega-datasets stocks.csv) silently dropped every point and the chart rendered blank.	Pre-convert temporal x-values to epoch-ms via a shared toEChartsTemporal() helper.
2	ECharts heatmap	Cell labels showed raw full-precision floats (e.g. 30.46666700000002) and string-sliced them when narrow, producing overlapping, illegible text.	Always round to a width-appropriate number of decimals, fall back to integer, hide only if it still overflows.
3	ECharts boxplot	Number(null) === 0, so rows with a missing measurement injected a spurious value at 0 (bogus whisker / outlier at 0).	Drop null/empty cells before numeric coercion.
4	Chart.js bubble	Same null->0 coercion piled bubbles at the origin (0, 0).	Drop rows with missing x/y before building points.

Testing

• npm run test -w packages/flint-js - 371 tests pass (3 new regression test files added).
• npm run typecheck -w packages/flint-js - clean.
• npm run build -w site - succeeds (galleries unaffected).
• Re-rendered all four scenarios to PNG and visually confirmed the fixes.

Notes / out of scope

• Vega-Lite radar does already emit a series legend in source (not reproducible); no change made.
• The canvasSize hard-ceiling overflow observed in the bug-bash is layout-level and intentional in compute-layout; any actual PNG overflow lives in the MCP renderer and carries high gallery-regression risk, so it is left for a separate, focused change.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com