test(cua-driver): restore shared fixture journals (conv2 1b/7) - #2181

Summary

Correction to the typed E2E foundation in #2179. The reviewed landing plan abbreviated the cross-platform app paths as fixtures/shared/electron/** and fixtures/shared/tauri/**, while the real sources live under fixtures/apps/cross-platform/**. As a result, the first extraction missed four files.

• injects the fixture journal URL into Electron through a context-isolated preload bridge
• forwards child-window journal state and preserves deterministic inactive-window behavior
• bundles the preload in the cross-platform macOS/Linux Electron build
• injects the same journal URL into the Tauri WebView initialization script

Windows build.ps1 remains owned by PR 2; Linux launcher-linux.sh remains owned by PR 4.

Validation

• node --check on Electron main and preload scripts
• bash -n on the Electron build script
• Tauri compilation reached generate_context!; standalone cargo check then stopped only because the build helper had not staged its generated ../web frontend directory
• the failed macOS shared run proved the missing journal bridge directly: AX was ready, but the independent fixture journal never received WEB_HARNESS_MARKER_v1

Source: #2161. Landing plan: frozen snapshot document.