fix(scripts): give the render-smoke overlap test room for process startup

scripts/lib/render-smoke.test.mjs

@@ -93,10 +93,33 @@ test("a paint landing just under the deadline is not failed by the render timer"
       `setTimeout(() => { console.log(${JSON.stringify(MARKER)}); ` +
         `setInterval(() => {}, 1000); }, ${paintAt});`,
     ),
-    // Deliberately tight: the render deadline expires while the survival window
-    // is still open, which is the whole shape of the bug.
-    timeoutMs: paintAt + 150,
-    surviveMs: 400,
+    // The shape being reproduced is an overlap, and only an overlap:
+    //
+    //   actual paint  <  timeoutMs  <  actual paint + surviveMs
+    //
+    // i.e. the render deadline expires while the survival window is still
+    // open. Both margins are deliberately wide, because the two clocks start
+    // at different moments: `paintAt` is measured from the child's first JS
+    // tick, while `renderTimer` is armed in the parent at spawn. The left
+    // margin therefore has to absorb everything in between — child process
+    // startup, plus whatever scheduling delay the machine is under.
+    //
+    // `stub()` spawns `process.execPath` directly, so there is no `script(1)`
+    // in this path: the PTY wrapper lives in `smoke-tui.mjs`, the caller. Node
+    // boot alone measured 46-59ms idle here. That fits the old `paintAt + 150`
+    // budget with room to spare, which is exactly why this read as a flake
+    // rather than a bug — but under the load of a full `local:gate` run the
+    // remaining ~90ms of slack is not enough to cover scheduling delay on both
+    // the child's boot and the parent's timer, and it failed three gate runs
+    // in a row while passing every time standalone (#2177). The diagnostic it
+    // failed with, "did not render", is the misreport this test exists to
+    // catch.
+    //
+    // Do not tighten these back up to make the test faster: the overlap is the
+    // point, not the tightness. 1200ms clears any plausible paint time, and
+    // still falls inside a survival window that cannot close before ~1750ms.
+    timeoutMs: 1200,
+    surviveMs: 1500,
   });
   assert.equal(r.code, 0, `expected pass, got: ${r.message}`);
   assert.doesNotMatch(r.message, /did not render/);