clean: v-gui notices and test CI logs

[GGRei]

Summary

Small hygiene pass for v-gui.

It cleans up V notices reported by CI without changing runtime behavior, and makes the existing CI Run tests steps easier to read on Linux, macOS, and Windows.

Changes

• Acknowledge unused parameters in v-gui code while preserving existing function signatures.
• Keep dock_layout_tree immutable/copy-on-write behavior while removing immutable-root notices.
• Add focused regression coverage for dock layout tree operations.
• Make CI Run tests steps print the tracked test file count and [i/N] test file list before running the existing v test gui/.

Tests

git diff --check
v fmt -verify -inprocess .
VJOBS=1 v -no-parallel test .