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>

src/main.rs

@@ -17,6 +17,7 @@ cfg_if::cfg_if! {
         use panic_probe as _;
     } else if #[cfg(any(feature = "heltec_v3", feature = "heltec_v4"))] {
         use esp_backtrace as _;
+        use esp_println as _; // provides defmt global_logger
     }
 }