cua-driver(macos): per-session overlay windows so each cursor sits z+1 above ITS window

Problem

With multiple concurrent sessions (multi-agent — see #1798), each session has its own agent cursor, but they're all drawn into one shared overlay NSWindow. A single window can occupy only one z-band, so the overlay is pinned just above the last-active cursor's target window (acknowledged in cursor/overlay.rs:556-557: "a single NSWindow can occupy only one z-band, so the last-active cursor's target wins").

Consequence: a cursor that belongs to a background window can render on top of a foreground window — the cursor isn't z-correct relative to its own window.

Desired

Each session's cursor should sit at z(its session's target window) + 1 — visible above its own window, but correctly occluded by windows stacked above it.

Proposal

Replace the single shared overlay window with one borderless, click-through overlay window per session/cursor:

• create on the session's first cursor command, destroy on session_end / idle-TTL;
• render that cursor into its own surface (sized to the cursor's bounding region, not the whole screen);
• orderWindow: NSWindowAbove relativeTo: <that session's target wid> so it tracks its own window's z-band independently (the PinAbove(wid) plumbing + per-session keys already exist).

Single-cursor behavior is unchanged. Isolated to platform-macos/src/cursor/overlay.rs.

Follow-up to #1798 (caller-declared session identity + multi-agent). Windows/Linux have no overlay, so macOS-only.

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