fix: expose mesh-level shader defines to the fragment shader

[mvaligursky]

Object/mesh-level shader defines derived from MeshInstance shader defs were only declared in the vertex shader, so user chunk overrides could not branch on them in the fragment shader (issue #8815).

Changes:

• Mesh-level defines (INSTANCING, SKIN, BATCH, MORPHING/MORPHING_INT/MORPHING_POSITION/MORPHING_NORMAL, SCREENSPACE) are now exposed to both the vertex and fragment shaders.
• LitShader: added a sharedDefineSet() helper (mirrors fDefineSet) that writes to both the vertex and fragment define maps, and routed the object-level defines through it.
• ShaderMaterial generator: extracted the duplicated mesh-define logic into a single addSharedDefines() method, now applied to both the vertex and fragment definitions.
• PIXELSNAP remains vertex-only (a transform-stage option, not a mesh property).

This is purely additive: these defines are only consumed by built-in vertex chunks today, so no existing shader output changes — the symbols simply become available to user chunk overrides (e.g. #if INSTANCING in diffusePS).

Fixes #8815

[Copilot]

Pull request overview

This PR fixes a limitation in shader generation where mesh/object-level shader defines (derived from MeshInstance shader defs) were only emitted for the vertex stage, preventing user fragment chunk overrides from branching on them (fixes #8815).

Changes:

• Exposes mesh/object-level defines to both vertex and fragment stages for LitShader programs.
• Refactors LitShader define emission via a new sharedDefineSet() helper to keep stage define maps in sync.
• Refactors ShaderMaterial program generation to centralize mesh/object-level define emission and apply it to both vertex and fragment definitions.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/scene/shader-lib/programs/shader-generator-shader.js	Adds a shared define helper so ShaderMaterial-generated programs expose mesh/object defines in both VS and FS.
src/scene/shader-lib/programs/lit-shader.js	Adds sharedDefineSet() and routes mesh/object-level defines through it so fragment shaders can see them too.

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