feat(kasan): add Kernel Address Sanitizer (KASAN) support for ESP-IDF

Add KASAN support for detecting heap memory safety bugs (buffer
overflows, underflows, use-after-free) at runtime using compiler
instrumentation and shadow memory. Gated behind
CONFIG_IDF_EXPERIMENTAL_FEATURES, with touch points kept to esp_system
and heap so other components stay untouched.

- Core runtime (esp_system/kasan.c, esp_kasan.h): nibble-based shadow
  memory in DRAM, poison/unpoison, per-access validation, and __asan_*
  stubs; hot-path stubs in IRAM so they stay valid with the flash cache
  off. Shadow init runs before heap bring-up.
- Heap integration (heap/heap_kasan*.c): alloc/free hooks add redzones,
  a quarantine FIFO, and shadow updates.
- Panic handling: disable checks once at the panic handler entry so
  backtrace and stack dumps can read redzones without nested reports.
- Build system: -fsanitize=kernel-address for app code, with HAL, SoC,
  esp_rom, SPI flash, esp_hw_support, bootloader_support, FreeRTOS, and
  heap internals excluded from instrumentation.
- Test app (tools/test_apps/system/kasan_test): Unity tests for
  overflow, underflow, use-after-free, and all sized __asan_* stubs,
  with halt and no-halt configurations.
- Docs: document KASAN in the heap memory debugging guide (EN and CN).

components/esp_system/system_init_fn.txt

@@ -24,6 +24,10 @@ CORE:  10: init_show_cpu_freq in components/esp_system/startup_funcs.c on BIT(0)
 CORE:  20: init_show_app_info in components/esp_app_format/esp_app_desc.c on BIT(0)
 CORE:  21: init_efuse_show_app_info in components/efuse/src/esp_efuse_startup.c on BIT(0)
 
+# When KASAN is enabled, initialise the KASAN shadow region before the heap allocator.
+# The shadow offset must be set up before the first heap_caps_init hook fires.
+CORE:  98: init_kasan_shadow in components/heap/heap_kasan.c on BIT(0)
+
 # Set the standard stream to non blocking and register the default vfs
 CORE:  99: init_vfs_linux_coop in components/vfs/vfs_linux_default_coop.c on BIT(0)