Deepen the config reader: entries(scope, projectDir) #48

Part of #44 — spec item 5.

The config reader's interface is entries(scope, projectDir), returning candidates with path, leniency, writability, and parsed entries — one interface answering "which config candidates exist for this scope base, what do they contain" for both parse modes. Both jsonc strategies live behind it: offset-preserving (for the splicer) and lossless (for readers).

Today's config-entries module dissolves into it.