Improve debug/trace diagnostics for mapping resolution

[sjaakd]

Problem
When a mapping key doesn’t match, it’s hard to see why (wrong JSONPath? missing key? unexpected value?). This slows down troubleshooting.

Why Java
You already have dcat.trace.enabled in root config; richer structured tracing belongs in exporter.

Proposed fix

• Trace logs: for each mapped node, log the resolved raw value, transformation (if any), map hit/miss, produced IRI, and whether node was emitted/skipped.
• Include focus node identifiers to correlate with SHACL messages.
• Explanation of the Error codes