fix: restore only visible iterator windows

[sit-d]

Summary

• Restore only the graph windows that were marked visible in the saved session record.
• Keep a safe startup fallback: if an old/empty session has no visible windows, tile the default window set so the app still opens with UI.

Verification

• Ad-hoc verification: /tmp/hermes-verify-iterator-restore-6kgzay4z.py asserted the startup restore block is gated by g_start_session.windows[i].visible, retains show_graph_window, falls back to tile_windows(NULL) when no windows are visible, removes the old unconditional GRAPH_COUNT restore loop, and runs git diff --check.
• Cleanup: removed /tmp/hermes-verify-iterator-restore-6kgzay4z.py after it exited 0.

Notes / Limitations

• Full Windows build/runtime verification was not run from the Linux cron environment because the repository build targets Win32/GDI via a Visual Studio tools prompt.
• This is separate from the existing generated-artifacts PR and only touches session-window restore behavior.

[sit-d]

Fresh ad-hoc verification after the final committed diff:

• /tmp/hermes-verify-iterator-visible-1eche7mf.py checked the startup restore block in iterator_gdi.c for the saved g_start_session.windows[i].visible gate, retained show_graph_window, zero-visible tile_windows(NULL) fallback, bounded graph-count loop, and removal of the old unconditional all-window restore loop.
• The verifier also ran git diff --check HEAD~1..HEAD.
• Result: exited 0.
• Cleanup: removed /tmp/hermes-verify-iterator-visible-1eche7mf.py.

This is ad-hoc structural verification; full Windows Win32/GDI build/runtime verification is still not available from the Linux cron environment.