fix: resolve ESP32-S3 linker errors, all boards produce firmware

Three fixes:
- Add .cargo/config.toml with -Tlinkall.x for xtensa target (resolves
  ROM functions, interrupt handlers, memory layout)
- Add integrated-timers feature to embassy-executor for ESP32 boards
  (resolves _embassy_time_schedule_wake)
- Add `use esp_println as _` to pull in defmt global_logger (was being
  stripped as dead code because nothing referenced the crate)

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

Cargo.toml

@@ -47,12 +47,14 @@ heltec_v3 = [
     "dep:esp-hal",
     "dep:esp-hal-embassy",
     "dep:esp-println",
+    "embassy-executor/integrated-timers",
 ]
 heltec_v4 = [
     "dep:esp-backtrace",
     "dep:esp-hal",
     "dep:esp-hal-embassy",
     "dep:esp-println",
+    "embassy-executor/integrated-timers",
 ]
 rak_wisblock_4631 = [
     "dep:cortex-m-rt",