type_density(): add echo.bw for reporting the smoothing bandwidth - #672

Closes #287.

Adds an echo.bw argument to type_density() for reporting the smoothing bandwidth and the number of observations behind it, neither of which is visible from the curve.

tinyplot(~Sepal.Length | Species, iris, type = type_density(echo.bw = "sub"))

Destinations are "sub", "xlab", "cap", and "cat" (console, with "print" as a synonym), in any combination. TRUE is shorthand for "sub"; the default FALSE reports nothing.

Following your two calls in the issue: all bandwidths are reported, and the argument sits on type_density() rather than tpar(). The "cap" and "cat" destinations are the ones you suggested.

I took the formatting conventions from the code you linked, including the version that was removed in #284: sprintf("%.4g"), bracketed lists collapsed to three entries plus an ellipsis, and "Joint Bandwidth" when the bandwidth is shared. Output:

N = [50, 50, 50]   Joint Bandwidth = 0.1809                # joint.bw = "mean"
N = [50, 50, 50]   Bandwidth = [0.1229, 0.2124, 0.2073]    # joint.bw = "none"
N = 150   Bandwidth = 0.2736                               # single group
N = [100, 100, 100, ...]   Bandwidth = [0.3875, ...]       # more than three

That same code answered a question I had been about to ask: a destination the user has already labelled is left alone, matching if (is.null(dots[["xlab"]])) in your original. So sub = "..." supplied by the user survives echo.bw = "sub".

Two notes on the implementation:

• The per-group lapply() is split into a lapply() that computes the densities and a Map() that reshapes them, so the bandwidths are still in hand when the label is built. To confirm this is a pure refactor, I rendered six configurations to SVG against main and diffed them: byte-identical with echo.bw off.
• match_echo_bw() and format_echo_vec() are internal helpers, so NAMESPACE is unchanged.

The "Titles" section said the x-axis title omits the observation count and bandwidth, which echo.bw = "xlab" now contradicts, so it points at the new argument.

Tests: 16 non-snapshot assertions covering argument normalization, the two error paths, the display formatting, and the reported text for joint, individual, and single-group cases. I could not run the snapshot tests locally (macOS), so CI will need to confirm those.

One point where I departed from the first mock-up, and where a word from you would settle it. That mock-up calls type_density(echo.bw = 'sub'), so joint.bw is at its "mean" default, and the subtitle lists [0.1229, 0.2124, 0.2073]. Those are the three individual bandwidths, but under joint.bw = "mean" none of them is the bandwidth actually used: all three curves are drawn with their weighted mean, 0.1809. Reporting the individual values there seemed likely to suggest the groups were smoothed differently when they were not, so the rule I implemented is to report what was used: one value, named as joint, whenever the bandwidth is shared, and the full list only when the groups genuinely differ.

That reading also matches your answer in the issue, since "all of them" was in response to the case where bandwidths differ across groups. Both mock-ups reproduce exactly under it: joint.bw = "none" gives [0.1229, 0.2124, 0.2073], and joint.bw = "full" gives 0.2736. If you would rather the individual bandwidths always be shown, that is a one-line change.