M11-02: End-to-end MVP workflow

[DrunkOnJava]

Created from the roadmap audit in TODO.md.

Source: https://github.com/DrunkOnJava/rvt-rs/blob/main/TODO.md#L1183
Tracking commit: 1ec8105

Scope

Labels: priority:P0, type:feature, area:product

Workflow:

• User opens viewer.
• User drops a supported .rvt.
• Viewer parses locally.
• Viewer shows levels and model elements.
• User can click wall/door/window/room and inspect fields.
• User can export IFC.
• IFC opens in IfcOpenShell/BlenderBIM with typed geometry-bearing
  elements.
• User can export diagnostics for bug reports.

Acceptance criteria:

• Playwright test covers the workflow with a demo fixture.
• CLI and Python can reproduce the same decoded counts.
• README has a short "supported MVP workflow" section.

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• The TODO acceptance criteria for this section are implemented or explicitly superseded by a linked decision.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