Add shadowCascadeMask to the render component

[mvaligursky]

Exposes the per-mesh directional shadow cascade mask on the render component, so selecting which cascades an entity casts into no longer requires walking its mesh instances from a script. Fixes #9126.

Changes:

• RenderComponent#shadowCascadeMask is assigned to every mesh instance the component manages, including ones created later by an asset load or a type change.
• The property is part of the component's property list, so it is initialized from addComponent data and preserved when an entity is cloned.
• Added render component tests covering the default value, initialization from component data, propagation to existing and to newly created mesh instances, and cloning.

API Changes:

• Added RenderComponent#shadowCascadeMask, defaulting to SHADOW_CASCADE_ALL. Combine SHADOW_CASCADE_0 .. SHADOW_CASCADE_3 to select individual cascades, and note that this only filters the meshes per cascade - use castShadows to disable shadow casting completely.

// only cast shadows into the two cascades closest to the camera
entity.render.shadowCascadeMask = pc.SHADOW_CASCADE_0 | pc.SHADOW_CASCADE_1;

ModelComponent is deliberately left alone as it is legacy, and the gsplat component cannot support this in unified mode, where shadow casters are registered per placement and rendered through a single shared mesh instance.

[github-actions]

Public API report

This PR changes the public API surface (+2 / −0), per the docs' rules (@ignore / @Private / undocumented are excluded).

Show API diff

+RenderComponent.get shadowCascadeMask(): number
+RenderComponent.set shadowCascadeMask(value: number)

Informational only — this never fails the build.

[github-actions]

Build size report

This PR changes the size of the minified bundles.

Bundle	Minified	Gzip	Brotli
playcanvas.min.js	2369.6 KB (+0.3 KB, +0.01%)	608.7 KB (+0.1 KB, +0.01%)	472.6 KB (−0.1 KB, −0.02%)
playcanvas.min.mjs	2366.9 KB (+0.3 KB, +0.01%)	607.6 KB (+0.0 KB, +0.01%)	471.9 KB (−0.3 KB, −0.06%)

[mvaligursky]

Automated PR review by Codex (GPT-5).

No actionable issues found.

Reviewed the change from the merge base for correctness, public API compatibility, component initialization and clone ordering, primitive and asset-driven mesh replacement, directional-shadow semantics, batching interactions, performance, documentation, and test coverage. The component-level value is retained independently of the current mesh instances and is consistently propagated through the shared meshInstances setter, so later primitive recreation and render-asset mesh updates inherit it. Adding the property to the ordered system property list also covers initialization and cloning without changing existing defaults.

Local verification on the exact head commit: the focused RenderComponent test suite passed (8 tests), ESLint passed for all changed files, generated TypeScript declarations include the new getter/setter, test:types passed, and git diff --check passed. All current GitHub checks are green.