feat(pdftract-2u6q2): implement diagnostic infrastructure
Add DiagnosticsCollector type for thread-safe diagnostic aggregation,
add hint field to DiagnosticJson, add missing error codes
(IMG_SOURCE_MIXED, PROFILE_INVALID, REPAIR_RESCUED_FROM_BACKWARDS_XREF),
and create comprehensive diagnostics documentation.
Changes:
- DiagnosticsCollector: Arc<Mutex<Vec<Diagnostic>>> wrapper with emit()
helpers for emitting diagnostics from multiple threads
- DiagnosticJson: add hint: Option<String> field for suggested actions
- DiagCode: add ImgSourceMixed, ProfileInvalid, RepairRescuedFromBackwardsXref
- docs/integrations/diagnostics-codes.md: comprehensive code catalog
Closes: pdftract-2u6q2
2be802aca5