chore: add counts keyword to stats/base/dists/poisson/ctor

Add the `counts` keyword to `package.json` to align with the
within-namespace majority pattern: 12 of the 14 sibling packages in
`stats/base/dists/poisson` carry `counts` (85.7% conformance), with
`mgf` the other outlier. The Poisson constructor instantiates a
distribution that models event counts in a fixed interval; the
omission is unintentional metadata drift.

Inserted between `univariate` and `discrete` to keep `counts` adjacent
to `discrete`, matching the sibling pattern used by `mean`, `variance`,
`mode`, and the other summary-statistic packages.

Metadata only; no source, behavior, test, or documentation changes.

Author: claude

lib/node_modules/@stdlib/stats/base/dists/poisson/ctor/package.json

@@ -68,6 +68,7 @@
     "properties",
     "props",
     "univariate",
+    "counts",
     "discrete"
   ]
 }