Three changes covering the hot update path for x/y changes on simple
nodes (no rotation/scale/mount):

1. CoreNode.update Global block: when both the node is simple and the
   parent's global is translate-only, the resulting global is also
   translate-only and collapses to two adds on tx/ty — bypassing the
   6-field Matrix3d.copy + 2 mul-add translate. Tracked with a per-node
   _globalIsTranslate flag that propagates down the tree. RTT chains
   conservatively opt out.

2. updateLocalTransform simple path: localTransform is guaranteed to be
   identity-shape (ta=1, tb=0, tc=0, td=1), so only tx/ty need to change
   between frames. _localIsTranslate tracks whether the matrix is still
   in identity-shape so a non-simple -> simple transition correctly
   restores it.

3. updateBoundingRect: fix `||` -> `&&` on the axis-aligned-fast-path
   check. With `||`, a matrix with one zero shear component and the other
   non-zero (e.g. tb=0, tc=-0.5) wrongly took the diagonal fast path and
   produced incorrect bounds. Now matches calculateRenderCoords semantics.

Adds setTranslate() helper to Matrix3d for the in-place tx/ty + mutation
flag write used by both #1 and #2.

Adds 12 unit tests covering simple-path reuse, non-simple -> simple
transitions, translate-only propagation through grandchildren, fast-path
opt-out when parent has rotation, and the bounding-rect axis check.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>