Add heap pressure management for BLE and rendering

Prevent OOM aborts by shedding BLE stack when free heap drops below 24KB before rendering. Lower BLE start threshold from 100KB to 80KB to match steady-state heap levels (~84KB). Reserve vector capacity upfront in page parsing to avoid reallocation crashes on fragmented heaps. Gate background section builds on heap availability.

src/activities/reader/EpubReaderActivity.cpp

@@ -269,9 +269,12 @@ void EpubReaderActivity::loop() {
   // RenderLock and locked out page turns. The build follows the reader instead, and instant
   // reopen comes from suspendBuild() persisting the laid-out pages as a partial on exit.
   // Skip while the render mutex is busy so we never delay a pending render; re-check
-  // isBuilding() under the lock since render() may have just finished it.
+  // isBuilding() under the lock since render() may have just finished it. Also skip
+  // while free heap is below the floor — the tick is deferrable, and parsing into a
+  // starved heap abort()s (see BACKGROUND_BUILD_MIN_FREE_HEAP).
   if (section && section->isBuilding() && !RenderLock::peek() &&
-      static_cast<int>(section->pageCount) < section->currentPage + BUILD_WINDOW_AHEAD) {
+      static_cast<int>(section->pageCount) < section->currentPage + BUILD_WINDOW_AHEAD &&
+      ESP.getFreeHeap() >= BACKGROUND_BUILD_MIN_FREE_HEAP) {
     RenderLock lock;
     // Re-check under the lock: render() (which also holds the RenderLock) may have finalized the
     // build between the outer isBuilding() check and acquiring the lock here, in which case
@@ -898,6 +901,20 @@ void EpubReaderActivity::render(RenderLock&& lock) {
     ~LifecycleUnpause() { bleinput::setLifecyclePaused(false); }
   } lifecycleUnpause;
 
+  // Shed the BLE stack before rendering into a starved heap. Everything below —
+  // page deserialization, glyph caching, catch-up build steps — allocates through
+  // throwing paths that abort() on OOM under -fno-exceptions. Field data: with no
+  // shed, a session ground to <2.2 KB free (per-glyph SD fallbacks, no AA, 4.7 s
+  // pages) and then aborted on a tiny vector growth. Freeing BLE returns ~52 KB and
+  // restores a large contiguous block; the lifecycle restarts it behind its heap
+  // gate once the pressure passes.
+  if (BleHid.isRunning() && ESP.getFreeHeap() < RENDER_MIN_FREE_HEAP) {
+    LOG_ERR("ERS", "Render heap %u below floor %u; freeing BLE RAM", (unsigned)ESP.getFreeHeap(),
+            (unsigned)RENDER_MIN_FREE_HEAP);
+    bleinput::setLifecyclePaused(true);
+    bleinput::stop();
+  }
+
   const auto showPendingSyncSaveError = [this]() {
     if (!pendingSyncSaveError) return;
     pendingSyncSaveError = false;