[v2/Linux] Verify GTK4/WebKitGTK 6.0 correctness follow-ups in PR #4570

Summary

Audit and resolve the concrete correctness findings recorded during the v3 GTK4/WebKitGTK 6.0 port that may also apply to the still-open v2 WebKitGTK 6.0 work in #4570.

This list was migrated from the repository's retired local beads tracker (wails-webview2gtk6-t4e.21). File names and line numbers referred to an early implementation and will have drifted; each item must be checked against the current PR rather than applied mechanically.

Findings to verify in #4570

• menu maps accessed without synchronisation;
• action lookup paths ignoring the ok result of map loads;
• a C.CString(title) allocation not being freed;
• a mismatched sendShowInspectorMessage declaration/signature;
• GtkWindow incorrectly cast as GdkToplevel;
• incorrect IsMinimised logic;
• drag-and-drop paths only printed rather than delivered through the API;
• separator handling missing from menus.

Acceptance criteria

• Check each finding against the current head of #4570.
• Fix confirmed defects in the PR or split them into independently tracked work.
• Mark obsolete/non-applicable findings with a short explanation.
• Add relevant race, lifecycle, menu, and resource-management tests before merge.

Related: #4570, #3193.