refactor(editor): size drawing through the tracked canvas transform - #503

Follows #502.

Summary

• Canvas tracks the transform from its current drawing units to logical screen pixels, through withTransform, save/restore, translate, scale and rotate.
• Nothing converts pixels to drawing units any more (pxToUpm is gone). Geometry is drawn in the current units; widths, dashes and radii are screen pixels applied in screen space.
  • Canvas.stroke() strokes the current path after resetting to pixels.
  • strokePath maps the Path2D to the screen with addPath(path, transform).
  • Circles, CPU handles, snap crosses and the read-only lock icon project their anchors with toScreen and draw in pixels.
  • Strokes and chrome therefore stay the right size inside any node transform, including scaled text runs.
• Pixel-sized drawing is enforced by type. withScreenSpace((screen, project) => …) hands its callback a branded ScreenCanvas and a projection from the outer units, and the screen-only helpers (handle and anchor drawing, the bounding-box handles) require a ScreenCanvas and ScreenPoints. Calling them from node units no longer compiles. That mistake had made anchor diamonds scale with zoom.
• Handles read the units-to-screen matrix and the culling bounds from the canvas (Canvas.visibleBounds). This removes the Editor.screenTransform and Editor.visibleLocalBounds plumbing, plus the unused Editor.screenToSceneDistance.
• The selection bounding box draws its rect and corner handles directly in screen space (Canvas.withScreenSpace), so it no longer offsets a scene rect by pixel distances converted at the current zoom.
• The handle marker shader derives each marker's screen rotation from the matrix, replacing the hard-coded -rotation that assumed a Y flip.
• Handle cull padding is cached per theme instead of rebuilt every frame.

Walking the node tree for drawing is left to the text-run work: sceneTransform already places child nodes correctly, and per-pass child dispatch belongs with the run's design.

Issue

Refs #129
Refs #458

Testing

• pnpm check (lint, typecheck, browser check, strict dead code, unit tests) passes, plus the pre-commit suite on each commit.
• pnpm format:check passes.
• Not yet checked in the app. Needs a visual pass on handle markers (direction, first and last shapes, GPU and CPU fallback), segment hover/selection strokes, metric guides and the read-only lock, snap crosses, the selection box and its component corner handles, and handle culling at the canvas edges.
• E2E not run locally. Relying on PR CI, especially the canvas snapshot specs in glyph-rendering and tools.

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