STEP Import as: hierarchy / flat / union + progress

[trailcode]

Summary

• Replace Import Union shapes checkbox with Import as: Preserve hierarchy (default), Flat solids, Union shapes.
• Show an Importing... modal during STEP Transfer; desktop gets stage/progress + Cancel on a background thread; web paints a simple wait dialog then runs Transfer on the main thread.
• Slim the Import dialog (no pre-import metadata Transfer) and document the modes in usage/CHANGELOG/module docs.

Closes #237

Test plan

• Desktop: Import a multi-body STEP with each Import as mode; verify Shape List hierarchy/names/root placement
• Desktop: Cancel during Transfer; confirm nothing added and cancel toast
• Desktop: Union failure aborts with no document changes
• WASM: Confirm Importing... shows before UI freeze; import completes
• PLY import unchanged
• Review docs/usage.md and CHANGELOG.md

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Note

Medium Risk
Changes the STEP import path (threading, cancel, and three Shape List placement modes) where geometry and hierarchy bugs would be user-visible; desktop off-thread OCCT transfer needs correct commit-on-UI-thread discipline.

Overview
STEP import UX is reworked: the Import dialog drops the pre-transfer metadata table and replaces the Union shapes checkbox with Import as (Preserve hierarchy, Flat solids, Union shapes).

Confirming import runs STEP through a new Importing... modal. Desktop runs prepare_step_import on a background thread with Atomic_progress_indicator (stage, percent, Cancel); WASM shows the modal for two frames then runs transfer on the main thread without Cancel. Document changes happen only via commit_step_import on the UI thread.

Occt_view::import_step now takes Step_import_mode and splits work into prepare_step_import (read/scale/fuse) plus commit_step_import (hierarchy groups, flat root leaves, or union under current group). XCAF/plain STEP readers forward Message_ProgressRange for cancellable transfer.

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