chore: bump Go toolchain to 1.26.3 (#9)

Pin Go version to 1.26.3 across go.mod, CI workflows, and Dockerfiles
for canonical alignment with the rest of the luxfi/* stack.

.github/workflows/ci.yml

@@ -14,7 +14,7 @@ jobs:
     - name: Setup Go
       uses: actions/setup-go@v5
       with:
-        go-version: '1.26'
+        go-version: '1.26.3'
 
     - uses: actions/cache@v4
       with:
@@ -34,7 +34,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go: ["1.26", "1.25"]
+        go: ['1.26.3']
 
     steps:
       - uses: actions/checkout@v4
@@ -68,7 +68,7 @@ jobs:
       - name: Setup Go
         uses: actions/setup-go@v5
         with:
-          go-version: '1.26'
+          go-version: '1.26.3'
 
       - uses: actions/cache@v4
         with: