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/.cargo/config.toml

@@ -0,0 +1,19 @@
+[build]
+target = "xtensa-esp32s3-espidf"
+
+[target.'cfg(target_os = "espidf")']
+linker = "ldproxy"
+runner = "espflash flash --monitor"
+rustflags = [ "--cfg",  "espidf_time64"]
+
+[unstable]
+build-std = ["std", "panic_abort"]
+
+[env]
+MCU = "esp32s3"
+ESP_IDF_VERSION = "v5.5.3"
+ESP_IDF_TOOLS_INSTALL_DIR = "workspace"
+
+# Uncomment this if you have moved the target dir by setting CARGO_TARGET_DIR or similar.
+# Required for now due to embuild limitations.
+#CARGO_WORKSPACE_DIR = { value = "", relative = true }