openapi: the $dynamicAnchor memo is a parameter of 72 functions and is read by two #982

Problem

*schema.AnchorIndex, the memo behind $dynamicRef expansion, is a parameter of 72 production functions: 48 in schema and 24 in operation. Two of them read it, soleAnchorSite and dynamicHop. The other 70 pass it on, and run in compilers/openapi/openapi.go owns it as a local so that it can.

lowering.Ctx already holds three memos that copies share on purpose: defsReader, typePositions and reached. #172 planned this one as a Ctx accessor too (AnchorsNamed(name)). It was kept out because building it reports a diagnostic for a bound, which is reported once, at first use. A Ctx method can return that diagnostic the way AnchorIndex.sites does now.

A trial moved the memo into Ctx (morphic @201bb745). The walk had to go to a new leaf package, because lowering may not import nodeview or yaml. go build ./..., go vet ./compilers/..., go test ./... (36 packages) and internal/archtest passed. The diff was +30 production, +50 test and +14 archtest lines. Removing a parameter rewrites the lines that carry it and deletes none, and the new package, accessor and tests cost more than the parameter saves. The trial exposed two couplings:

• TestLoweringCallGraph_ReadsEveryPackageThatLowers finds the packages the recursion pin reads by looking for a lowering.Ctx parameter. The moved walk has none, so it silently leaves the pin unless the package is listed by name.
• TestValueKindSwitches_NameEveryValueKind treats "reaches ir" as "sees ir.ValueKind". A package that reaches ir only through ids fails its require.

The files involved have no non-comment change between @201bb745 and @3a706a7b, and the parameter counts hold at both. The line counts are from @201bb745.

Direction

A draft. Do not land it alone: it is a signature simplification that costs lines. #981 rewrites the same signatures to drop the hint parameter, and doing both in one pass pays for the rewrite once. If #981 is not taken, leave this as it is.

If it is taken: a leaf package for the walk and the memo, a Ctx.DynamicAnchorSites(name) that returns the sites and the once-only bounds diagnostic, the package named in the recursion pin, and the ValueKind test reading "sees" instead of "reaches".

Out of scope

Bundling lowering.Ctx, *compile.Types, the memo and depth into one struct. Ctx is deliberately a value, and a type under compilers/ may carry at most 20 methods.

Related: #981, #172, #454

Priority: P4, rule 1 (housekeeping that unblocks nothing). git grep -nE '\.sites\(' -- compilers ':!*_test.go' → 2 lines, both in schema/schema.go (morphic @3a706a7b).