fix(common): parented position offsets ignore the parent's rotation and scale

[stormmuller]

Summary

composeWithParent in src/common/systems/transform-system.ts inherits rotation and scale correctly, but composes a child's world position by plain addition. The child's local offset is never rotated or scaled by the parent's world transform, so a child of a rotating parent spins in place instead of orbiting it.

Raised by @stormmuller while reviewing #580.

Current behaviour

src/common/systems/transform-system.ts:

world.rotation = parent.world.rotation + local.rotation      // correct
world.scale    = parent.world.scale    * local.scale         // correct
world.position = parent.world.position + local.position      // missing two terms

A correct 2D TRS composition transforms the child's local offset by the parent before adding it:

world.position = parent.world.position
               + rotate(local.position * parent.world.scale, parent.world.rotation)

Reproduction

Parent at the origin with rotation.local = Math.PI / 2, child parented to it with position.local = { x: 10, y: 0 }:

	Expected	Actual
child position.world	(0, 10)	(10, 0)
child rotation.world	π/2	π/2 ✓

The child's rotation is right, so the sprite visibly spins — it just doesn't orbit. The scale case fails the same way: parent scale.local = (2, 2) and child position.local = (10, 0) gives (10, 0) where it should give (20, 0).

Symptom in a real scene: a turret parented to a rotating tank rotates correctly but stays glued at an un-rotated offset, so it detaches from its mount as the tank turns.

Why it wasn't caught

transform-system.test.ts has seven tests and none of them attach a RotationEcsComponent or ScaleEcsComponent to a parent. Nothing currently asserts this behaviour in either direction.

Related dead code

parent-position-system.ts, parent-rotation-system.ts, and parent-scale-system.ts (all in src/common/systems/) carry the same additive position composition. They are not exported from src/common/systems/index.ts and are referenced only by their own test files — they appear to be superseded by createTransformEcsSystem. Worth deleting alongside the fix rather than fixing in three places, unless they're being kept deliberately.

Suggested scope

• Fix composeWithParent to apply the parent's world rotation and scale to the child's local offset.
• Add tests covering: rotated parent, scaled parent, rotated + scaled parent, and a three-deep chain.
• Decide whether to delete the three superseded parent-*-system.ts files and their tests.

Note this is a behaviour change for any existing content that parents a positioned entity to a rotated or scaled one, and that has been authored against the current (incorrect) composition. Given the engine is pre-1.0, correcting it seems right, but it likely wants a changelog entry flagging the visual change.