test: close the two coverage tails on the plan surfaces

The plan unit's same-reference gate for a repeated identical /plan selection
(the one uncovered branch in plan-mode/index.ts) gains an explicit case, and
ui-plan's empty node-half apply is exercised the way sibling surface plugins
do (the ui-conversation no-op precedent). Both files read 100% across all
four axes under the CI coverage lane's per-file thresholds.

packages/plan/plan-mode/tests/projection.spec.ts

@@ -73,6 +73,9 @@ describe('plan projection unit', () => {
     const bench = await harness(true)
     runPlanCommand(bench.session, '', 0)
     expect(bench.values().plan).toEqual({ active: false, pending: true })
+    // A repeated identical selection returns the same state reference (no frame).
+    runPlanCommand(bench.session, '', 1)
+    expect(bench.values().plan).toEqual({ active: false, pending: true })
     commitPlanMode(bench.session, true, 0)
     expect(bench.values().plan).toEqual({ active: true, pending: false })
   })