Split the config editor class: reader / detector / splicer #47

Part of #44 — spec item 4.

Split the config editor class by responsibility into reader / registration detector / surgical splicer modules:

• the splicer's interface stays exactly as is (it already passes the depth test);
• the detector becomes single-purpose;
• the reader feeds both;
• tests stop constructing an editor to read.

Internal seams only — external behavior unchanged; tests keep hitting external behavior.