Table aggregation, chart tooltips, and the calcs rename (0.10.0)

[dankor]

BREAKING: measures: is now calcs:, dashboard: is now layout:, and a chart's measure: is calc:. The old names are not accepted — an existing dashboard must be updated.

Charts

• Table gains columns, measures and sort: it groups and aggregates, or stays row-by-row when measures is empty. Measure cells and column headers carry the same hover card the other charts use, and clicking a row crossfilters on its whole combination of dimension values.
• Bar gains top, direction and an optional y, plus a paged legend, an automatic time grain with a picker, and windowing for a long axis.
• Pie gains a paged legend with clickable rows.

Calcs

• A column calc may carry name/description to relabel a raw column wherever it is displayed, without renaming what the YAML refers to. This needed a rule change: a self-reference in a formula now means the dataset column.
• str2dt() parses text dates, including timezone offsets.
• A null grouping key is one group, not one row per measure.

Fixes

• Chart tooltips anchor to real CSS boxes (an anchor-name on an SVG shape silently fails), are opaque and sized to their text, and clear the shape they describe. A stale duplicated block in bar/chart.css had been overriding the rebuilt bar tooltip entirely.
• A chart that cannot render shows an error card instead of leaving the cell spinning forever — htmx only swaps a 2xx, so an exception was invisible.
• An embedded table keeps its calcs through search and pagination; it re-renders its own cell rather than calling a route that cannot see the calcs: block.
• A header's or separator's toolbar buttons are clickable again.

Sample and docs

• files/orders.csv regenerated to 120 rows via a checked-in generator, shaped so the demo can show grain switching, legend paging, null groups and a row-level formula.
• The starter dashboard is now a commented quick guide across three tabs.
• New guards: chart CSS coherence (duplicate selectors, unbalanced braces, literal markup in comments) and route-level tests for the htmx wiring.

504 tests pass.