Hide Tools-panel hover-blue expand bar

Goal

Make the vertical dodge-blue bar that appears on the right edge of the fixed-width Tools panel disappear permanently so the panel no longer advertises a resize affordance that is functionally disabled.

Root cause

In hcie-gui-egui/src/app/mod.rs:2573-2617 the egui_dock separator style is configured with the active accent colour for color_hovered and color_dragged. At the boundary between the locked 36 px Tools column and the main content it renders a 4 px bar that turns blue on hover. The column is pinned (layout_solver.lock_tools_column + Tools => TOOLBOX_WIDTH in dock.rs), so resize is impossible; the coloured separator is purely cosmetic and unwanted.

Changes

File: hcie-egui-app/crates/hcie-gui-egui/src/app/mod.rs
Range: ui_main_assembly separator-style block (~L2570-2617)

Replace the existing colour assignments:

style.separator.color_idle    = splitter_idle;
style.separator.color_hovered = splitter_hover;
style.separator.color_dragged = splitter_hover;

with fully-transparent values and remove the drag affordance:

style.separator.width = 4.0; // visual width -- zeroing is safe but keep 4px to avoid layout shift
style.separator.extra_interact_width = 0.0; // no invisible drag target since resize is locked anyway
style.separator.extra = 175.0;
style.separator.color_idle    = egui::Color32::TRANSPARENT;
style.separator.color_hovered = egui::Color32::TRANSPARENT;
style.separator.color_dragged = egui::Color32::TRANSPARENT;

The now-unused locals splitter_idle and splitter_hover (~L2573-2574) that were only fed into the separator block can be removed in the same pass, or left in place since they have no further effect.

Constraints / non-goals

Do NOT change the 36 px Tools column width or the lock_tools_column logic in layout_solver.rs.
Do NOT touch any engine/Layer 1-4 code (not relevant here).
Do NOT introduce new dependencies or touch egui_dock source.

Validation

cargo check -p hcie-gui-egui -- confirm no type errors.
Run the application and visually verify that hovering the right edge of the Tools panel no longer produces a blue bar.
Squeeze/expand the main window to confirm the 36 px Tools column stays fixed and no stray scrollbar or resize artefact appears (the extra_interact_width = 0.0 change means the separator has no drag zone; column width is still enforced by lock_tools_column).

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