feat(editor): add adjustable pen smoothing

[jondkinney]

Freehand strokes now use adjustable release-time smoothing, with a medium 3/6 level by default. The live stroke follows the raw pointer input; 0/6 preserves that exact path, while higher levels clean up the finished path without moving its endpoints.

Alt+wheel changes smoothing for the selected stroke, or sets the level for the next stroke when nothing is selected. The setting remains undoable and is preserved in the operation log.

Tested with make check.

[Copilot]

🟡 Changes recommended

Alt+wheel has two behavioral regressions involving rectangle selection feedback and pen-default adjustment while another layer remains selected.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Adds adjustable, undoable post-stroke smoothing to the freehand pen.

Changes:

• Implements bounded RDP/Chaikin smoothing levels.
• Adds editor controls, persistence, transforms, and rendering support.
• Adds documentation and end-to-end smoke coverage.

File summaries

File	Description
src/stroke-smoothing.hpp	Declares smoothing levels and API.
src/stroke-smoothing.cpp	Implements bounded stroke smoothing.
src/editor.hpp	Stores the active smoothing level.
src/editor.cpp	Integrates smoothing with editing and history.
src/capture.hpp	Adds raw geometry and smoothing metadata.
src/capture.cpp	Persists and renders smoothed strokes.
tests/stroke-smoothing-smoke.hpp	Declares the smoke test.
tests/stroke-smoothing-smoke.cpp	Tests smoothing and editor integration.
tests/editor-smoke.cpp	Registers the new smoke test.
README.md	Documents smoothing controls.
CMakeLists.txt	Adds implementation and tests to builds.

Review details

• Files reviewed: 11/11 changed files
• Comments generated: 2
• Review effort level: Balanced

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[Copilot]

🟡 Changes recommended

Synchronous smoothing can stall the UI, and loaded smoothing levels are not range-validated.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

Suppressed comments (1)

Previously missed (1) — in code that hasn't changed since the last review.

src/editor.cpp:5114

• This smoothing pass runs synchronously inside mouseReleaseEvent. The adversarial path can leave all 2,048 samples for the iterative RDP pass, which still has quadratic worst-case work, so releasing a zig-zag stroke can stall the visible overlay contrary to docs/threading.md:5-8. Please move the pass to a QtConcurrent/QFutureWatcher flow or use an algorithm with a frame-bounded worst case before committing the stroke.

• Files reviewed: 11/11 changed files
• Comments generated: 1
• Review effort level: Balanced

[Copilot]

🟢 Approval recommended

The implementation is bounded, consistently integrated, documented, and covered across its primary behavioral paths.

Review details

• Files reviewed: 12/12 changed files
• Comments generated: 0 new
• Review effort level: Balanced