M5-04: Add export diagnostics sidecar

[DrunkOnJava]

Created from the roadmap audit in TODO.md.

Source: https://github.com/DrunkOnJava/rvt-rs/blob/main/TODO.md#L727
Tracking commit: 1ec8105

Scope

Labels: priority:P1, type:feature, area:ifc, area:cli

• Add --diagnostics out.json to export commands.
• Include:
  • input metadata
  • decoded element counts
  • exported element counts
  • skipped elements
  • unsupported features
  • warnings
  • confidence summary

Acceptance criteria:

• CLI and viewer can generate/share diagnostics for bug reports.
• Diagnostics schema is documented.

Definition of done

• The TODO acceptance criteria for this section are implemented or explicitly superseded by a linked decision.
• Tests, fixtures, or documentation are added at the level appropriate to the change risk.
• User-facing behavior and limitations remain honest in README/docs/viewer messaging.
• Relevant CI checks pass before the issue is closed.

[DrunkOnJava]

Starting implementation now. Planned scope: add a stable JSON diagnostics schema for IFC export, wire rvt-ifc --diagnostics <path>, expose the same diagnostics payload to WASM/Python entry points, document the schema, and add regression coverage around exported counts, suppressed low-confidence candidates, warnings, and CLI output.

[DrunkOnJava]

Implemented in 0bbe5a6.

Summary:

• Added stable schema v1 export diagnostics with input metadata, decoded/exported counts, skipped low-confidence candidates, unsupported features, warnings, and confidence summary.
• Added rvt-ifc --diagnostics <path> across default, placeholder, and diagnostic-proxy export modes.
• Exposed diagnostics to WASM via openRvtBytesWithDiagnostics(bytes) and Python via RevitFile.export_diagnostics_json() / rvt_to_ifc_diagnostics(path).
• Added docs in docs/export-diagnostics.md and updated README, Python, viewer API, and status docs.
• Added integration coverage for CLI sidecar writing and diagnostics schema/shape.

Validation:

• cargo test export_diagnostics_sidecar_reports_default_ifc_readiness --test walker_to_ifc_integration
• cargo test rvt_ifc_cli_writes_diagnostics_sidecar --test rvt_ifc_diagnostics_cli
• cargo test --test walker_to_ifc_integration --test rvt_ifc_diagnostics_cli
• cargo check --lib --features wasm --no-default-features
• cargo test -p rvt-py
• tools/quality.sh --quick