openapi: split operations.go by construct #1057

Problem

compilers/openapi/internal/operation/operations.go (1,378 lines) holds five constructs beside the operation itself:

• the path-item walk and how a path item mounts;
• tag grouping;
• responses and status keys;
• callbacks;
• webhooks.

paramledger.go is a helper only params.go uses. pathitemref_beside_test.go tests pathitemref.go alongside pathitemref_test.go.

Direction

• pathitem.go: paths, path items, mounting, and the path-item fields an operation inherits.
• groups.go: tags and service grouping (it pairs with the existing groups_test.go).
• responses.go: responses, status-key parsing, error cases.
• callbacks.go: callbacks and webhooks.
• params.go gains parameter merging and the parameter ledger; paramledger.go goes.
• operations.go keeps LowerService and the lowering of one operation.
• Tests follow their names; pathitemref_beside_test.go merges into pathitemref_test.go. The three TestContent_* tests in operations_test.go stay until openapi: split content.go by construct #1059 splits content.go, because content_test.go is busy until then.

Blocked by

#1046 edits operations_test.go, params.go, params_test.go and groups_test.go.

Done when

• Every top-level declaration of the package, with its doc comment and any comment between declarations, is byte-identical before and after the move (compared with a throwaway go/ast script), and no golden changes.
• Comments that name a file name the file the code now lives in, changed in a commit of their own.
• go test ./compilers/openapi/... passes without -update.

Priority: P4, rule 1 (housekeeping: moves code between files and changes no lowering, validation, IR or emit-contract behaviour). wc -l compilers/openapi/internal/operation/operations.go → 1378 (morphic @a1f15b29).