v4 ram: PSRAM malloc threshold 1024 -> 256 (+5.4 KB total, no T-Deck regression)

Measured on the V4: post-spiffs 167496 -> 171316, pre-iface 121432 ->
126836. Verified on the T-Deck (8 MB PSRAM, the working reference) that
pushing ordinary allocations to PSRAM breaks nothing: boots, chats and the
store all fine, so the threshold is safe for every board.

Session total on the V4: ~12 KB internal reclaimed (6.7 KB statics + 5.4 KB
threshold).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main.cpp

@@ -300,7 +300,7 @@ void setup() {
   // the threshold at runtime so ordinary mallocs land in PSRAM; anything that
   // genuinely needs internal/DMA memory asks for it by capability and is
   // unaffected. Tunable — raise it if something misbehaves.
-  heap_caps_malloc_extmem_enable(1024);
+  heap_caps_malloc_extmem_enable(256);
   BOOTMEM("post-extmem");
 
   // Widen the task-watchdog grace period. The ~5 s default trips during a legitimate-but-slow flash