Efficiency tab: polish the unavailable line and the empty

Found during Task 7167's visual check of Audit › Sessions › Efficiency on a local stack, where analytics is unconfigured. 1. The one-line "Where the time goes · <reason>" state sits flush against the sub-tab underline, with no top spacing. Give it the same top gap the card states get. 2. When the Project memory panel has no sessions in the window ("No sessions in this window."), it still draws the full Read-rate table with every row at "0 units / none injected". Hide the table, or collapse it to one line, when the window is empty. Out of scope: the Efficiency reason sentences; the `unavailable_kind` picker logic (Task 7167). Done means: - A screenshot on the live stack shows both fixes at desktop width. - A web test covers the empty-window Project memory state. - `./dc check full` green.

Efficiency unavailable line now clears the sub-tab rule (pt:1); empty-window Project memory hides an all-zero read-rate table, with a web test. Visual check done at 1280px. PR #5849 merged by lane (full gate). Follow-up draft 7203.