Invalid `floor` invocation from `isTimeInterval`

Somewhere during binning of temporal data this function is executed:

https://github.com/observablehq/plot/blob/356f579b1d947ee05a914420eddff0f29cee300a/src/options.js#L428-L430

Notice that no argument is passed to floor when the function tries to figure out if the result type is a Date.

This is in conflict with the type definitions, which indicate that a parameter to floor is guaranteed:

https://github.com/observablehq/plot/blob/356f579b1d947ee05a914420eddff0f29cee300a/src/interval.d.ts#L34-L47

I believe the simplest fix would be

-   */
-  floor(value: T): T
+   *
+   * If no value is passed, the function is expected to return *any* result
+   * matching its return type.
+   */
+  floor(value?: T): T

Related: #2423

Edit: I just noted that this behaviour is documented on the Bin transform page (but not the Interval transform), which increases my confidence that it’s really just the type that’s off.

Time intervals are intervals that are also functions that return a Date instance when called with no arguments.

Start with the floor call in src/options.js and compare it with the floor signature in src/interval.d.ts. Update the type contract to reflect the no-argument invocation, then verify that the declaration matches the documented binning behavior and type checking succeeds.