Fix material readiness to gate on light texture readiness

[Created by Copilot on behalf of @bghgary]

Problem

SpotLight projection textures and IES profile textures were not gated by material readiness checks. When a projection texture was still loading, prepareLightSpecificDefines would set PROJECTEDLIGHTTEXTURE to false, the shader would compile without the projection effect, and the material would report isReady() = true. This caused scene.executeWhenReady() to fire before the projection texture loaded, producing incorrect rendering (e.g. intermittent visual-test failures in BabylonNative).

Root cause

• SpotLight.prepareLightSpecificDefines() treats unloaded projection/IES textures as "not present" rather than "not ready".
• No readiness gate existed for light texture resources in the material pipeline.
• scene.isReady() → mesh.isReady() → material.isReadyForSubMesh() all returned true despite the texture still loading.

Fix

1. Light: adds areLightTexturesReady(): boolean (default true) for subclasses that use texture resources.
2. SpotLight: overrides areLightTexturesReady and checks _projectionTexture.isReadyOrNotBlocking() and _iesProfileTexture.isReadyOrNotBlocking() (respects BaseTexture.isBlocking semantics, matching how other material textures are gated).
3. AreLightsTexturesReady helper (new, in materialHelper.functions.ts): iterates mesh.lightSources up to maxSimultaneousLights, respects scene.lightsEnabled and disableLighting, and mirrors the light selection used by PrepareDefinesForLights / BindLights.
4. Materials (StandardMaterial, BackgroundMaterial, PBRBaseMaterial, OpenPBRMaterial, NodeMaterial): each calls AreLightsTexturesReady(...) in isReadyForSubMesh and returns false early if any selected light is not ready. For PBRBaseMaterial and OpenPBRMaterial the gate is placed before _prepareEffect() so we don't compile and cache a throwaway shader while textures are still loading.
5. Node blocks (LightBlock, PBRMetallicRoughnessBlock): when an explicit this.light is bound, the block's isReady now checks this.light.areLightTexturesReady(). LightBlock registers itself as a blocking block when it has an explicit light, so the check participates in NodeMaterial._sharedData.blockingBlocks.

When a projection/IES texture finishes loading, the existing _markMeshesAsLightDirty() callback in SpotLight's projectionTexture / iesProfileTexture setters triggers re-evaluation: the helper returns true on the next readiness pass, the material reports ready, and scene.executeWhenReady() proceeds.

Tests

packages/dev/core/test/unit/Lights/babylon.spotLight.test.ts adds three regression tests:

1. Generic contract: scene.isReady() stays false while any light reports its textures are not ready (monkey-patches light.areLightTexturesReady).
2. SpotLight projectionTexture: scene.isReady() stays false while the projection texture is not ready.
3. SpotLight iesProfileTexture: scene.isReady() stays false while the IES profile texture is not ready.

All three fail on master and pass on this branch. See #18336 for the test-only variant that demonstrates the master failure directly in CI.

Related

• test: SpotLight projection texture readiness regression test (expected to fail on master, companion to #18255) #18336 — test-only PR against master showing the failure.