Document the current public Arena boundary

probe.py

@@ -115,11 +115,10 @@ ENVS = Path(__file__).parent / "environments"
 def discover() -> dict[str, list[Path]]:
     """Taskset id -> the package directories that ship it, read off the manifests.
 
-    The seven names this file used to carry as a literal tuple are `environments/*/pyproject.toml`
+    The names this file once carried as a literal tuple are `environments/*/pyproject.toml`
     now: an environment is gated by existing, not by being remembered here. One directory may
-    declare more than one taskset — the Tera bridge ships four spatial tasksets out of a single
-    package — through an optional `[tool.arena] tasksets = [...]`. Absent that key the taskset id
-    is the project name, which is what all seven of the originals do.
+    declare more than one taskset through an optional `[tool.arena] tasksets = [...]`. Absent
+    that key the taskset id is the project name, which is what all eight public packages do.
 
     Other sessions create directories under `environments/` while this runs, so a manifest that
     disappears between the glob and the read is skipped rather than fatal. A manifest that is