feat: add scrollable UI transform and UiScrollResult

[kuruk-mm]

Summary

Proposes adding scrollable UI containers and a programmatic way to observe / control their scroll state. Verified against the reference renderer in bevy-explorer (crates/scene_runner/src/update_world/scene_ui/mod.rs, crates/ui_core/src/scrollable.rs), which already wires these fields end-to-end.

What this adds

PBUiScrollResult — new component (ecs_component_id = 1202)

Renderer → scene. The renderer writes the viewport's current scroll offset back to the scene whenever the user scrolls (wheel, drag, or programmatic move), letting scene logic react to scroll state.

message PBUiScrollResult {
  decentraland.common.Vector2 value = 1; // current scroll offset (h, v)
}

ui_transform.proto additions

ShowScrollBar enum — which scroll bars are visible on a scrollable node:

• SSB_BOTH (default) — horizontal + vertical
• SSB_ONLY_VERTICAL
• SSB_ONLY_HORIZONTAL
• SSB_HIDDEN — no bars (content still scrollable via wheel/drag)

ScrollPositionValue message — oneof:

• position (Vector2) — literal scroll offset
• reference (string) — element_id of a descendant; the renderer scrolls the viewport so that element is visible

Three new optional fields on PBUiTransform:

Field	#	Purpose
element_id	74	Unique name for this UI element (HTML-id-like). Registered in a named-element table so other components/RPCs can refer to it by name. Today consumed by ScrollPositionValue.reference and the UI focus RPC. Default: empty.
scroll_position	75	Scroll offset of this node's scroll viewport. Only meaningful when overflow = YGO_SCROLL. Default: position=(0,0).
scroll_visible	76	Which scroll bars to render. Default: SSB_BOTH.

Proposed usage

1. Make a node scrollable

UiTransform.create(entity, {
  width: 300, height: 200,
  overflow: YGOverflow.YGO_SCROLL,     // required for scroll to engage
  scrollVisible: ShowScrollBar.SSB_ONLY_VERTICAL,
})

2. Set scroll position imperatively

UiTransform.createOrReplace(entity, {
  overflow: YGOverflow.YGO_SCROLL,
  scrollPosition: { value: { $case: 'position', position: { x: 0, y: 250 } } },
})

3. Scroll a named child into view

UiTransform.create(childEntity, { elementId: 'section-3', parent })

UiTransform.createOrReplace(parent, {
  overflow: YGOverflow.YGO_SCROLL,
  scrollPosition: { value: { $case: 'reference', reference: 'section-3' } },
})

4. React to user scrolling

const result = UiScrollResult.getOrNull(entity)
if (result?.value) {
  // result.value is the current { x, y } scroll offset
}

Test plan

• make buf-lint passes
• make buf-build passes
• Confirm downstream SDK codegen picks up the new symbols
• Renderer (bevy-explorer / Unity) tested against a scene exercising literal scroll, reference scroll, and PBUiScrollResult read-back

[github-actions]

Test this pull request

• The @dcl/protocol package can be tested in scenes by running

  npm install "https://sdk-team-cdn.decentraland.org/@dcl/protocol/branch//dcl-protocol-1.0.0-26197255415.commit-323091f.tgz"