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/BleInput.h

@@ -25,11 +25,22 @@ namespace bleinput {
 // Advertised central name shown to peripherals during pairing.
 inline constexpr const char* kHostName = "CrossPoint";
 
-// Heap floor for starting the NimBLE stack (~57 KB) while leaving the reader's
-// section-build pre-flight (40 KB) satisfiable afterwards. Shared by the main-loop
-// lifecycle gate and the reader menu's toggle (which offers a defrag restart when a
-// user turns BT on below the floor).
-inline constexpr size_t kStartMinFreeHeap = 100 * 1024;
+// Heap floor for starting the NimBLE stack (measured begin() cost: ~52 KB). This must
+// be reachable in steady-state reading, not just at fresh boot: with an SD font and a
+// loaded section, mid-session idle heap is ~84 KB — a 100 KB floor was only ever
+// passed via a stale-gate bug (heap sampled before the render lock), and once that
+// was fixed the stack could never start again after a shed. 80 KB starts at ~84 KB
+// steady state and leaves ~32 KB of reading slack; the render shed floor
+// (RENDER_MIN_FREE_HEAP, 24 KB) backstops the tight sessions, and the lifecycle
+// cooldown paces any shed/restart cycle. Shared by the main-loop lifecycle gate and
+// the reader menu's toggle (which offers a defrag restart below the floor).
+inline constexpr size_t kStartMinFreeHeap = 80 * 1024;
+
+// Lower floor for the Bluetooth settings screen, where the user has explicitly asked
+// for BLE right now (scanning/pairing is dead without the stack). No page renders or
+// section builds run there, so the reader-sized reserve above doesn't apply — only
+// NimBLE's own ~57 KB plus working margin.
+inline constexpr size_t kStartMinFreeHeapExplicit = 70 * 1024;
 
 // Start the BLE HID host (idempotent). Returns false if BLE is compiled out or
 // NimBLE init failed. Safe to call repeatedly.