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/config.toml

@@ -0,0 +1,6 @@
+[target.xtensa-esp32s3-none-elf]
+rustflags = [
+    "-C", "link-arg=-nostartfiles",
+    "-C", "link-arg=-Tlinkall.x",
+    "-C", "link-arg=-Tdefmt.x",
+]

Cargo.lock

@@ -359,6 +359,8 @@ dependencies = [
  "critical-section",
  "document-features",
  "embassy-executor-macros",
+ "embassy-time-driver",
+ "embassy-time-queue-driver",
 ]
 
 [[package]]

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",

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
     }
 }