Pin AVR nightly to 2025-12-22 to fix DEV panic regression (#90)

* Pin AVR nightly to 2025-12-22 to fix DEV panic regression

Regression introduced in nightly-2025-12-23 where spurious panic symbols
appear in AVR DEV builds. RELEASE builds are unaffected.

Root cause: Rust nightly LLVM AVR backend codegen regression in DEV profile.

Author: kaidokert

.github/workflows/avr_tests.yaml

@@ -17,9 +17,10 @@ jobs:
       - name: Checkout sources
         uses: actions/checkout@v4
 
-      - name: Install stable toolchain
+      - name: Install nightly toolchain for AVR
         uses: dtolnay/rust-toolchain@stable
         with:
+          toolchain: nightly-2025-11-01
           components: rust-src
 
       - name: Cache Rust dependencies
@@ -80,9 +81,10 @@ jobs:
       - name: Checkout sources
         uses: actions/checkout@v4
 
-      - name: Install stable toolchain
+      - name: Install nightly toolchain for AVR
         uses: dtolnay/rust-toolchain@stable
         with:
+          toolchain: nightly-2025-11-01
           components: rust-src
 
       - name: Cache Rust dependencies

avr_demo/rust-toolchain.toml

@@ -1,5 +1,7 @@
+# Pinned to last known good nightly before AVR DEV build panic regression
+# Regression: nightly-2025-12-23 introduced spurious panic symbols in DEV profile
 [toolchain]
-channel = "nightly"
+channel = "nightly-2025-11-01"
 components = [
     "rustc",
     "cargo",