fix: restore ordinary window geometry and Windows startup position

[WilliamWang1721]

Result / 用户结果

Remember ordinary window size across full process exit, restore its Windows position on a still-attached display, and honor explicit Lua window.position. Explicit grid/position settings take priority over saved geometry. Refs #105, #151, #312; grid dimensions and Quick Terminal's independent sizing were already implemented and are not claimed as new fixes.

Design / 设计边界

Geometry shares the existing atomic session document. Only the first ordinary cold-start window consumes it; new windows, Quick Terminal and administrator isolation retain their own policy. Save normal bounds rather than maximized/fullscreen viewport dimensions, preserve an empty last-window snapshot without restoring closed shells, and clamp on the selected display. Optional JSON position/display fields keep old sessions compatible. Ownership and coordinate note.

Windows saves physical desktop position and logical size using the pinned GPUI normal-placement contract. Non-Windows position restoration is outside this change. Hidden Windows configuration placement waits until activation consumes GPUI's cached placement.

Evidence / 验证依据

• Fork Windows before/after run: the same registered-window snapshot test fails on main because geometry is absent; initial candidate compilation exposed API/borrow errors, subsequently corrected. This run is not a passing fix result.
• Updated fork proof: source f99fcbac, Windows tests pending at creation. Formatting, architecture against 9dd3d649, and platform branch gate passed. Head 0afbbde9 applies the exact remote formatting and tightens the branch budget.
• Regressions cover atomic multiwindow/empty-session roundtrips, explicit Lua configuration, legal large window sizes, offscreen fitting, registered-window snapshots, and real HWND outer/normal placement. Fractional DPI and top/left taskbar offsets are conversion tests; multi-monitor hardware and visual acceptance remain unverified. All execution is remote GitHub Actions.

Required Review / 必须确认

• Followed contribution, architecture and project constraints.
• Responsibilities and existing persistence authority are retained; no budgets were raised or tests removed.
• Final-head checks and native platform results complete.
• Platform and visual verification limits are stated; no new product text.

AI-assisted implementation: GPT-6.1 SOL. Reasoning effort was not verified. No merge or approval is requested from automation.