check:nav-reachable-mobile polls the rail instead of reading it once at 600 ms

plugin-studio's first frame after the ☰ tap lands at ~1.3 s on a phone, so the single read at 600 ms caught the
rail mid-slide (x = -4) and failed although it opened (measured on 443d2efda and f4066c36f alike). The gate now
reads every 50 ms for up to 2.5 s and passes on the first fully-on-screen frame. "On screen" also checks the
right edge: in RTL the closed rail parks at x = innerWidth, which x >= 0 alone counted as open.

internal/check-nav-reachable-mobile.mjs

@@ -178,7 +178,12 @@ for (const t of targets) {
                               n.getBoundingClientRect().width > 0)
     if (!btn) return { clicked: false }
     btn.click()
-    return new Promise(res => setTimeout(() => {
+    // POLLED, not read once. A slow page (plugin-studio: its first frame after the tap lands at ~1.3 s on a phone)
+    // was caught MID-SLIDE by a single read at 600 ms (x = -4) and failed although the rail opened. Read every
+    // 50 ms for up to 2.5 s and pass on the first fully-on-screen frame. "On screen" is the rail INSIDE the
+    // viewport on both edges: in RTL the closed rail parks at the RIGHT (x = innerWidth), which x >= 0 alone
+    // would count as open.
+    const readRail = () => {
       const s2 = new Set(); let rail = null
       ;(function w(n) {
         if (!n || s2.has(n)) return; s2.add(n)
@@ -187,8 +192,16 @@ for (const t of targets) {
         for (const c of (n.children || [])) w(c)
       })(document.body)
       const r = rail ? rail.getBoundingClientRect() : null
-      res({ clicked: true, railX: r ? Math.round(r.x) : null, railOnScreen: !!r && r.x >= 0 && r.width > 0 })
-    }, 600))
+      return { railX: r ? Math.round(r.x) : null, railOnScreen: !!r && r.width > 0 && r.x >= 0 && r.right <= window.innerWidth + 1 }
+    }
+    return new Promise(res => {
+      const t0 = performance.now()
+      ;(function poll() {
+        const now = readRail()
+        if (now.railOnScreen || performance.now() - t0 > 2500) return res({ clicked: true, ...now, ms: Math.round(performance.now() - t0) })
+        setTimeout(poll, 50)
+      })()
+    })
   })
 
   if (!after.clicked || !after.railOnScreen) {