fix(cua-driver): keep Windows cursor visible without focus theft

[f-trycua]

Summary

• make the un-targeted Windows cursor overlay reliably reach the front of the ordinary z-order band with a non-activating transient HWND_TOPMOST → HWND_NOTOPMOST transition
• preserve target-bound placement exactly between the target and an independent foreground window
• add a pure branch-selection regression plus hosted Windows coverage for move-only visibility, final non-topmost state, unchanged focus and real cursor, targeted effect isolation, and the existing no-leaked-input oracle

Fixes #2802

Verification

• cargo fmt --all -- --check
• git diff --check
• cargo test -p platform-windows --lib — 41 passed
• cargo test -p cursor-overlay --lib — 42 passed
• cargo test -p cua-driver --test agent_cursor_windows_test — compiles on the local non-Windows host; 0 Windows-gated tests run
• cargo check --target x86_64-pc-windows-msvc -p cua-driver --test agent_cursor_windows_test — local cross-check cannot complete because the macOS host lacks Windows SDK/MSVC C headers required by transitive native dependencies; the canonical interactive Windows lane is the authoritative compile and behavior gate

Hosted Windows certification

• exact source: 4f1e63bbfbd7b22ac16c2d0559f6a4934e32d2ba
• canonical native-lane dispatch: https://github.com/trycua/cua/actions/runs/30965712916
• windows-desktop-agent-cursor-px — PASS (Pixels, Focus, ZOrder, Cursor); proves visible un-targeted overlay at the top of the ordinary band, final non-topmost state, unchanged foreground and physical pointer, then target-bound placement between the WPF target and an independent Electron foreground window with target counter effect and no foreground leak
• windows-desktop-agent-cursor-no-input-leak — PASS (Focus, ZOrder, Cursor, NoLeakedInput)
• the cursor test binary completed 2 passed; 0 failed
• downloaded results.jsonl SHA-256: 7ee13e3ea9dbfbade0be1303d440a657d724a27054cad72da295d1ac5e98123c
• downloaded summary.md SHA-256: 6d980c825c05c3a0bf3288f84d797e5f8c1e107066b1b76d43cfb9fb946c5dba

The native lane's only failing cell was the unrelated existing windows-webview2-left-click-px-background: UIA desktop enumeration exceeded its 2-second deadline and the action fell back to global_input while that cell expected accessibility. That cell passed in the two preceding exact-head native runs (30963333853 and 30964551564); the final-head change only replaces the cursor test's target fixture. All 41 other final-run matrix cells, including both cursor cells, passed or produced their expected refusal.

Standard PR checks are green, including the bounded Nix unit-test retry. The installer compatibility summary was still queued at the time this evidence was attached.

This PR remains draft for final audit; it has not been graduated or merged.