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.toml

@@ -9,7 +9,7 @@ cfg-if = "1.0"
 critical-section = "1.2"
 defmt = "0.3"
 embassy-embedded-hal = "0.2"
-embassy-executor = { version = "0.6", features = ["task-arena-size-12288", "executor-thread"] }
+embassy-executor = { version = "0.6", features = ["task-arena-size-32768"] }
 embassy-sync = "0.6"
 embassy-time = "0.3"
 embassy-usb = { version = "0.3", features = ["defmt"] }
@@ -27,7 +27,7 @@ ssd1306 = { version = "0.10", features = ["async"] }
 embassy-futures = "0.1"
 
 # ESP32-S3 (Heltec V3) — requires esp toolchain (espup)
-esp-backtrace = { version = "0.14", optional = true, features = ["esp32s3", "panic-handler", "defmt"] }
+esp-backtrace = { version = "0.14", optional = true, features = ["esp32s3", "panic-handler", "exception-handler", "defmt"] }
 esp-hal = { version = "0.22", optional = true, features = ["esp32s3"] }
 esp-hal-embassy = { version = "0.5", optional = true, features = ["esp32s3"] }
 esp-println = { version = "0.12", optional = true, features = ["esp32s3", "defmt-espflash"] }
@@ -47,18 +47,24 @@ heltec_v3 = [
     "dep:esp-hal",
     "dep:esp-hal-embassy",
     "dep:esp-println",
-    "embassy-executor/integrated-timers",
+    "embassy-time/generic-queue-8",
 ]
 heltec_v4 = [
     "dep:esp-backtrace",
     "dep:esp-hal",
     "dep:esp-hal-embassy",
     "dep:esp-println",
-    "embassy-executor/integrated-timers",
+    "embassy-time/generic-queue-8",
 ]
 rak_wisblock_4631 = [
     "dep:cortex-m-rt",
     "dep:embassy-nrf",
     "dep:panic-probe",
     "embassy-executor/arch-cortex-m",
+    "embassy-executor/executor-thread",
 ]
+
+[profile.release]
+codegen-units = 1
+lto = "fat"
+opt-level = 3