No way to ask about a sub-period: add a date-range window the tables follow

[tschm]

The app draws the whole file, always. The first question two NAV series invite is about a
stretch of it — what did these two do in 2022 — and today the only way to ask is to leave
the app and refilter the parquet.

Everything needed is already in place. columns.date_column knows whether there is a date
axis or row numbers; aligned sorts by period; plot and stats both consume the same
aligned frame. What is missing is one input.

Shape. A range control in the middle column, under the rebase switch: mo.ui.date_range
when date_column(frame) is not None, mo.ui.range_slider over the row index when it is.
Filter once in the cell that already derives pair and chart, and let both the chart and
the two summary tables come off the filtered frame.

Why a range control and not an Altair brush. alt.selection_interval(bind="scales")
would be fewer lines, and it would break the repo's own invariant: zooming the axis moves
the picture and leaves the tables describing the full sample, which is the exact failure mode
of #75 reintroduced deliberately. A range input filters the data, so the tables follow and
"the numbers beside the chart always describe the lines in it" keeps holding. Getting a
brush to filter instead of zoom needs mo.ui.altair_chart for the round trip, which also
means line_chart stops being the plain Altair chart that plot.py's docstring promises —
worth knowing before reaching for it.

Two things fall out for free once the window exists:

• Rebasing indexes to the first period of the window, so "both at 100 from the start of
  2022" is what the switch means — which is the comparison the feature is for.
• The caption gains a reason to name the window, and the drawn-vs-sampled fix in The caption reports the aligned sample, not the thinned curve the chart draws #75 is the
  same sentence.

Sequencing: after #75. Building a window on a caption that already misreports its sample
means writing the fix twice.