[jqnatividad]

Stacked on #4841 (ternary), which is stacked on #4840 (barpolar). Retarget to master once those merge; this branch will need the same rebase onto master.

What's new

• viz funnel --area draws the funnel as a plotly.js funnelarea: stacked sections sized by each stage's share of the total, labeled with its value and percent of the total. The bar funnel labels the conversion from the previous stage instead, which has no meaning on a share-sized section.
  • Stage order (first appearance in the file), aggregation (--y sum, or row counts without it) and the negative-total check are shared with the bar funnel.
  • Growth warning: a funnel area's outline always narrows, so a stage larger than the one before it can't show as growth, which the bar funnel shows as a wider bar. --area still draws, but names the first such stage on stderr ("stage Renewed (260) is larger than Subscribed (200) before it … drop --area to see it as a wider bar"). The --area help says so too.
• --area and --degrees are rejected on every other chart, viz smart included. Both checks now run in run() before dispatch. Previously they lived in build_plot, which viz smart never reaches, so viz smart --area/--degrees silently ignored the flag (roborev 5472; --degrees comes from feat(viz): barpolar wind rose; clock-face polar bars for the smart seasonality panel #4840).
• Regenerated docs/help/viz.md and qsv-viz.json (--area, a usage example).

Gallery

A funnel --area figure next to the bar funnel, on the same signup data. On data this skewed, the last stages are thin slivers with crowded labels, which is why the bar funnel stays the default.

Verification

• cargo t viz -F all_features: 485 unit and 660 integration tests pass. New: viz_funnel_area_sizes_sections_by_share, viz_funnel_area_warns_when_a_stage_grows, viz_area_only_applies_to_funnel (pie and smart), plus smart --degrees in viz_barpolar_usage_errors.
• Mutation-checked: ignoring --area, changing its labels, removing the non-funnel check, suppressing the growth warning, warning on every pipeline, and exempting smart from either flag check each fails a test.
• scripts/double-run-check.py --check OK; cargo +nightly fmt; clippy reports no warnings on added lines.
• In Chrome: the gallery figure beside the bar funnel.

The wiki Visualization page update is staged locally and will be pushed after merge.

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