A selector or field XPath that fails to compile is now reported once up
front as a located error (identity constraint 'k': invalid selector/field
XPath '...'), instead of try? silently turning a schema author's typo into
a disabled constraint. The per-run query cache from #112 is the choke point:
compileErrors() probes every constraint's selector and fields in
deterministic order, the failures cache as nil, and the walk afterwards
treats those constraints as matching nothing, so the rest of the document
still validates. Tests cover a malformed field and a malformed selector.
Closes#111
Mihaela Mihaljevic
0edc1f77f9