Add a bounded per-model transcript body height cache keyed by the existing row render fingerprint, then use measured spans to render only the visible alt-screen item window instead of building the full body string every frame.
Mouse hit testing now uses the same visible item layout, while inline rendering and selection copy keep the full layout path for behavior parity.
Tests cover height cache reuse, visible-only item rendering, absolute selectable body coordinates, and output parity with the previous full-layout renderer.
Tested: env GOCACHE=/tmp/zero-go-cache go test ./internal/tui
Tested: git diff --check
Tested: /bin/bash -lc "GOCACHE=/tmp/zero-go-cache go test ./..."
