feat: Add dynamic layer opacity support to flame_tiled

[ufrshubham]

Description

flame_tiled supported reading layer opacity from .tmx files at load time, but there was no way to change a layer's opacity at runtime. This PR adds setLayerOpacity and getLayerOpacity methods to RenderableTiledMap, enabling users to dynamically adjust layer opacity after the map has loaded.

• RenderableLayer.opacity is converted from a late field (computed once at init) to a getter/setter pair. The getter returns the effective opacity (own × parent chain); the setter stores the own opacity and fires onOpacityChanged().
• A protected onOpacityChanged() hook is added to RenderableLayer. FlameTileLayer overrides it to rebuild its cached Paint object; GroupLayer overrides it to propagate the call recursively to all children so nested tile layers stay in sync.
• RenderableTiledMap gains setLayerOpacity(int layerIndex, {required double opacity}) and getLayerOpacity(int layerIndex).
• FlameImageLayer required no changes — it already reads opacity live on every render() call.

Checklist

• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• [NA] I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

Related Issues

Closes #3066

[spydon]

Lgtm!