M5-02: Remove misleading generic proxies from default export

[DrunkOnJava]

Created from the roadmap audit in TODO.md.

Source: https://github.com/DrunkOnJava/rvt-rs/blob/main/TODO.md#L695
Tracking commit: 1ec8105

Scope

Labels: priority:P0, type:bug, area:ifc

• Do not emit IFCBUILDINGELEMENTPROXY for low-confidence scan hits
  by default.
• Allow diagnostic exports to include them with provenance.

Acceptance criteria:

• Einhoven 2023 no longer emits HostObjAttr-* as normal model
  elements.
• Diagnostic mode can still show them.

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• The TODO acceptance criteria for this section are implemented or explicitly superseded by a linked decision.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]

Implemented in f4015d4.

What changed:

• Added DiagnosticRvtDocExporter for opt-in low-confidence IFCBUILDINGELEMENTPROXY diagnostic output.
• Kept RvtDocExporter conservative by default, so low-confidence HostObjAttr-* scan hits remain suppressed from normal IFC export.
• Added rvt-ifc --diagnostic-proxies with conflict handling against --placeholder.
• Added a corpus-backed regression that verifies diagnostic HostObjAttr-* proxies include Pset_RvtRsDiagnosticCandidate provenance and that default IFC export remains conservative.
• Updated README/status docs to distinguish production export from diagnostic proxy export.

Local verification:

• cargo test diagnostic_ifc_export_includes_hostobjattr_proxies_with_provenance --test walker_to_ifc_integration
• cargo test walker_to_ifc_einhoven_project_has_nonzero_elements --test walker_to_ifc_integration
• cargo test --test walker_to_ifc_integration
• cargo run --quiet --bin rvt-ifc -- --help
• tools/quality.sh --quick

Note: tools/quality.sh --quick passed; local cargo-audit was skipped because cargo-audit is not installed, and cargo-deny was skipped because the advisory database path is read-only in this environment.