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>

Justfile

@@ -47,7 +47,7 @@ clippy board:
     case "$target" in xtensa-*) \
         just _require_esp_toolchain; \
         [ -f "$HOME/export-esp.sh" ] && . "$HOME/export-esp.sh"; \
-        env="RUSTC=$(rustup which rustc --toolchain esp)"; extra="+nightly -Zbuild-std=core";; \
+        env="RUSTC=$(rustup which rustc --toolchain esp)"; extra="+nightly";; \
     esac; \
     eval $env cargo $extra clippy --target $target --features $feat -- -D warnings
 
@@ -81,7 +81,7 @@ _cargo board cmd:
     case "$target" in xtensa-*) \
         just _require_esp_toolchain; \
         [ -f "$HOME/export-esp.sh" ] && . "$HOME/export-esp.sh"; \
-        env="RUSTC=$(rustup which rustc --toolchain esp)"; extra="+nightly -Zbuild-std=core";; \
+        env="RUSTC=$(rustup which rustc --toolchain esp)"; extra="+nightly";; \
     esac; \
     eval $env cargo $extra {{cmd}} --target $target --features $feat