feat(kg-build): surface URL handler and mapping failures with typed result and sync report - #19

Summary

Problem

URL handler failures (HTTP 403s, YARRRML parse errors) were silently swallowed —
the cloud workflow exited 0 with no record of what failed.

What changed

Failure capture

• Introduced FailedUrl dataclass (timestamp, url, handler name, message) and
  KgImportResult (url count, failures, ok property, elapsed seconds) as the
  typed return value of KgImportWorkflow.run()
• DefaultUrlHandler now accumulates failures per handler instead of logging and
  discarding them
• rml_mapping.apply_mapping now re-raises on processing errors (YARRRML parse,
  missing dataset URI) instead of returning MappingResult(graph=None), so
  mapping failures propagate through the protocol into KgImportResult.failures
  alongside HTTP failures
• run_cloud_workflow and main.run_kg_import_workflow both raise
  SystemExit("Total URLs: X, Successes: Y, Failures: Z") on failure

Reporting

• New report_util module: render_as_markdown, render_as_csv, write_report
• When CloudWorkflowConfig.output_dir is set, writes graph_sync_report.md
  (always) and graph_sync_failures.csv (on failure only)
• Markdown report: summary table with total URLs, successes, failures, success
  rate, and execution time; top-10 error groups with count, stable key, clickable
  example URL, and full error message; overflow note when distinct error types
  exceed the limit
• CSV: timestamp, url, error per failed URL