chore(firmware): scaffold Spike 1 Blink crate

Generated from esp-rs/esp-idf-template for the ESP32-S3 std target.
src/main.rs toggles GPIO 2 every 500 ms and logs `blink N` over USB-
serial — the minimum bring-up surface called out in
docs/v0.1-mvp-technical.md (Spike 1: confirm toolchain, flash, and basic
GPIO). edition=2024 with rust-version=1.85.

No editor/render/git/usb/fs modules yet; those land per the spike
methodology when later spikes need them.

firmware/sdkconfig.defaults

@@ -0,0 +1,12 @@
+# Rust often needs a bit of an extra main task stack size compared to C (the default is 3K)
+# You might have to increase this further if you allocate large stack variables in the main task
+CONFIG_ESP_MAIN_TASK_STACK_SIZE=8192
+
+# Increase a bit these stack sizes as they are also a bit too small by default
+CONFIG_ESP_SYSTEM_EVENT_TASK_STACK_SIZE=4096
+CONFIG_FREERTOS_IDLE_TASK_STACKSIZE=4096
+
+# You might have to increase this further if you spawn your own Rust threads 
+# that allocate large stack variables; or better yet - use 
+# `std::thread::Builder::new().stack_size(XXX)` for spawning
+CONFIG_PTHREAD_TASK_STACK_SIZE_DEFAULT=4096