View roll (Shift+NumPad 4/6), settings, and pbf-to-png tooling

[trailcode]

Summary

Feature branch Trailcode/hot-keys vs main.

Compare on GitHub: main...Trailcode/hot-keys

3D view roll

• Shift+NumPad 4 / Shift+NumPad 6 roll the view around the screen Z axis (Blender-style), implemented in GUI::on_key (src/gui_mode.cpp) calling Occt_view::roll_view_z_deg.
• Settings includes a 3D view navigation section: View roll step (degrees per key press). Persisted as gui.view_roll_step_deg in settings JSON (default 45, clamped to 0.1–180).
• Help link from Settings opens usage.md#view-roll.

Tooling

• scripts/pbf-to-png.py (and scripts/pbf-to-png.ps1) rasterize PDF content to PNG; supports files mislabeled as .pbf via PyMuPDF.

Documentation

• Updates to usage.md, usage-settings.md, and usage-occt-view.md for view roll and related behavior.

Notes / housekeeping

• agents/issues/001-imgui-openpopup-id-stack-and-tables.md documents the ImGui ID-stack pitfall when using OpenPopup inside BeginTable (context: Settings UI work).
• Refactor/cleanup in src/sketch_underlay.h and small src/gui.cpp / console tweaks.

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Review checklist

• View roll feels correct at default and after changing View roll step (including Ctrl+click numeric entry on the slider).
• Settings round-trip: save/reload app and confirm gui.view_roll_step_deg.
• Emscripten build still runs if you ship wasm from this branch.
• Optional: run pbf-to-png.py on a sample .pbf/PDF asset.

Related

• See CHANGELOG.md [Unreleased] for user-facing bullets aligned with this work.