archtest: the recursion pin reads free functions only

[OmarAlJarrah]

loweringCallGraph in internal/archtest/recursion_test.go builds its graph from top-level functions with no receiver. Methods are skipped entirely.

Until #217 that was not quite true — it also read methods on the lowerer type, by receiver name. That type is gone, so the branch was dead and #210 removed it. What is left is a graph that cannot see a method at all.

What it misses today

anchorWalk.walk and anchorWalk.walkMapping in compilers/openapi/internal/schema/schema.go are mutually recursive: the mapping walk descends into values by calling walk, and walk dispatches mappings to walkMapping. That is a real recursion in the lowering, bounded by an explicit depth counter, and the pin says nothing about it.

Others in the same position, none of them currently recursive: serviceGroups.group/finalize, anchorIndex.sites.

Why it was left

#210 fixed the value-edge blind spot and its acceptance required the pinned sets to be unchanged, so that a real answer could be told from a re-baselined one. Adding methods changes them — anchorWalk becomes a fourth set — so it could not ride along without giving up that property.

What to decide

Whether a method belongs in this graph at all. The pin exists to say which lowerings cannot be moved or tested alone; anchorWalk's two methods are exactly that, so the answer is probably yes. If so:

• Record methods under a name that cannot collide with a free function's (the graph refuses collisions rather than resolving them, so walk on two receivers must stay distinguishable).
• Re-pin, with anchorWalk's set added and a note saying why it is allowed to be recursive.

Acceptance

• The pinned sets name every mutual recursion among the lowerings, methods included.
• Planting a method that closes a new cycle reddens the test.