Rename TrimNodeMutation to TrimGeneMutation

[chrxh]

Summary

• Renames the TrimNodeMutation struct/field/function family to TrimGeneMutation, matching the existing gene-level mutation naming convention (the mutation trims a random node from each gene, keyed by geneProbability, same pattern as DuplicateGeneMutation/DeleteGeneMutation).
• Covers descriptor structs (TrimNodeMutationDesc), TO structs, GPU kernel structs, serialization ids, the applyMutations_trimNode kernel function, the trimNodeMetaMutationsSigma parameter, GUI labels/settings keys, and test class/case names.
• Leaves the unrelated trimNodes(...) helper in GenomeDescEditService.cpp (genome-preview cell-count trimming) untouched — different feature, coincidental name overlap.

Test plan

• Build (blocked locally: external/vcpkg submodule not initialized in this environment)
• EngineInterfaceTests.exe, NetworkTests.exe, PersisterTests.exe
• EngineTests.exe --gtest_filter=TrimGeneMutationTests.*:MetaMutationTests.*trimGene*:AccumulatedMutationTests*

🤖 Generated with Claude Code

[chrxh]

Folded into #782 — combining both mutation renames into a single PR as requested.