feat(compare): migrate compare charts to echarts_timeseries_line and drop the legacy plugin

[rusackas]

SUMMARY

Tier 3 of #41714 (targets remove-legacy-viz-pipeline): retire the nvd3 Time-series Percent Change (compare) chart by migrating saved charts to the ECharts Line Chart (echarts_timeseries_line).

• New MigrateCompareChart processor — the shared TimeseriesChart base already handles the whole nvd3 timeseries control mapping (including normalizing the legacy absolute comparison type), so the subclass only declares the source/target viz types. Registered with the superset viz-migrations CLI and covered by a unit test.
• Alembic revision e5f6a7b8c9d0 (chained after the bubble migration) with the standard MySQL MEDIUMTEXT widening and reversible form_data_bak backups.
• Removes the nvd3 Compare plugin, its MainPreset/preset registrations, the NVD3CompareTimeSeriesViz backend class, and the cumulative-line branch of the shared NVD3 renderer.
• Behavior note for UPDATING.md: the nvd3 renderer's interactive re-basing (clicking a point to re-base percent change) is not preserved by the ECharts line chart.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

Saved Percent Change charts re-open as ECharts Line Charts.

TESTING INSTRUCTIONS

• pytest tests/unit_tests/migrations/viz/nvd3_compare_chart_to_echarts_test.py passes.
• npm run test -- plugins/legacy-preset-chart-nvd3 passes.
• Manual: superset db upgrade then open a formerly-compare chart — renders as an ECharts line chart; superset db downgrade restores the original form_data.

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided: proportional to the number of saved compare slices; no downtime expected
• Introduces new feature or API
• Removes existing feature or API

🤖 Generated with Claude Code

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Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (remove-legacy-viz-pipeline@ea90b59). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...nd/plugins/legacy-preset-chart-nvd3/src/NVD3Vis.ts	0.00%	1 Missing ⚠️
superset/cli/viz_migrations.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@                      Coverage Diff                      @@
##             remove-legacy-viz-pipeline   #41738   +/-   ##
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  Misses                                ?    50753           
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Flag	Coverage Δ
hive	39.20% <75.00%> (?)
javascript	69.63% <0.00%> (?)
mysql	57.85% <75.00%> (?)
postgres	57.91% <75.00%> (?)
presto	40.75% <75.00%> (?)
python	59.30% <75.00%> (?)
sqlite	57.49% <75.00%> (?)
unit	100.00% <ø> (?)

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