fix: resolve ESP32-S3 runtime crash, match official esp-hal examples

Root causes found and fixed:
- USB OTG init stole the PHY from USB-Serial-JTAG (the only USB on V4)
- executor-thread feature conflicted with esp-hal-embassy's executor
- Linker flags needed -Wl, prefix for GCC-to-LD passthrough
- Missing exception-handler feature on esp-backtrace
- Missing alloc in build-std
- Missing LTO and codegen-units=1 in release profile
- Vext on V4 is active LOW (datasheet correct, Arduino lib was wrong)

Now matches the official esp-hal v0.22.0 examples exactly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.cargo/config.toml

@@ -1,6 +1,9 @@
-[target.xtensa-esp32s3-none-elf]
+[target.'cfg(target_arch = "xtensa")']
 rustflags = [
+    "-C", "link-arg=-Wl,-Tlinkall.x",
+    "-C", "link-arg=-Wl,-Tdefmt.x",
     "-C", "link-arg=-nostartfiles",
-    "-C", "link-arg=-Tlinkall.x",
-    "-C", "link-arg=-Tdefmt.x",
 ]
+
+[unstable]
+build-std = ["alloc", "core"]