Web and terminal: native-preview cover for popups, a size-claim capability, and lab observations #358

Small web and terminal follow-ups from #349.

• On macOS desktop, Popover, Menu, Select and tooltips can draw under a native browser preview. Sheet, Dialog and AlertDialog take an occlusion lease, but useNativePreviewOcclusion hides the whole preview and has no rectangle. Add a lease that carries a rectangle so the preview hides only when the popup overlaps it (browserSurfaceStore, possibly the desktop bridge).
• The client treats the terminalUserInputClaim server capability as proof that the server has the typed-last size hand-over rule. A server that advertises one without the other would break this silently. Add a dedicated capability (packages/contracts/src/environment.ts, apps/server/src/lifecycle.rs).
• Observed in lab runs without a known cause, to investigate: the WebGL terminal renderer sometimes falls back to the DOM renderer within a run (probably after reloads); an empty over-tall grid shows between a terminal's mount and its first snapshot after a reload; terminal_command_activity_hint_decodes_strictly_without_entering_launch_env was flaky before its close path was fixed and needs repeated runs to confirm; LAN pairing in the dev build answered "Invalid pairing token" once.
• Not measured: whether the composer's model picker can show old values for one render on a rapid reopen (Worktrees: create dialog keeps the previous worktree's values #346).