ci(release): serialize fdroid_reference matrix (max-parallel:1) to avoid OOM contention

.forgejo/workflows/release.yml

@@ -100,6 +100,12 @@ jobs:
 
   fdroid_reference:
     strategy:
+      # Sequential, not parallel: 2-3 concurrent NDK/CMake/Gradle builds on the
+      # same host contend for RAM and get OOM-killed (silent truncated log, no
+      # "max run time" message) — confirmed 2026-07-21 when only the ABI that
+      # ended up running alone (after the other two died) completed. One at a
+      # time is slower (~30-35min x3) but fits comfortably in the 90m timeout.
+      max-parallel: 1
       matrix:
         abi: [armeabi-v7a, arm64-v8a, x86_64]
         include: