docs(fdroid): post-merge runbook for reproducible developer-signed builds

docs/fdroid/reproducible-post-merge.md

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+# F-Droid reproducible, developer-signed builds — post-merge follow-up
+
+The initial F-Droid submission is **F-Droid-signed** (F-Droid builds from source and
+signs with its own key). Making F-Droid publish *our* tane-upload–signed APK
+(same signature as Google Play, so users move between stores) is a **post-merge**
+step, because F-Droid derives `SOURCE_DATE_EPOCH` from the **commit timestamp of
+our metadata file inside fdroiddata** — a value that only exists once the MR is
+merged. So a byte-matching reference cannot be produced beforehand.
+
+## Why our own CI couldn't do this pre-merge
+
+- Deps are NOT the problem: F-Droid uses the **standard public repos**
+  (`mavenCentral()` + Google's maven) — no private mirror. `jsr305` and the old
+  jcenter artifacts are on Maven Central. The build environment that "just works"
+  is the public image `registry.gitlab.com/fdroid/fdroidserver:buildserver-trixie`
+  (bundles the Android SDK + NDK); we verified a full build succeeds in it.
+- The real blocker is `SOURCE_DATE_EPOCH` (fdroidserver `get_source_date_epoch`
+  falls back to the fdroiddata metadata commit, since the app isn't checked out
+  yet when it's read). Unknown until F-Droid commits our metadata.
+
+## Post-merge procedure
+
+Run this **after** `org.comunes.tane` is merged into fdroiddata, from a clone of
+the **merged** fdroiddata (so the metadata commit — and thus SOURCE_DATE_EPOCH —
+matches what F-Droid used). Requires Docker and the tane-upload keystore.
+
+```bash
+#!/bin/bash
+set -eu
+IMG=registry.gitlab.com/fdroid/fdroidserver:buildserver-trixie
+FDROIDDATA=/path/to/merged/fdroiddata     # git clone of gitlab.com/fdroid/fdroiddata (or fork synced past merge)
+KS=/home/vjrj/proyectos/sync/comunes/shared-l2/apkSigning/tane-upload.jks
+VC_BASE=10                                  # pubspec versionCode base (0.1.x+N -> N)
+
+docker run --rm -v "$FDROIDDATA":/build -v "$KS":/tmp/ks.jks:ro \
+  -e KSPASS -e KEYPASS -w /build "$IMG" bash -c '
+    set -e
+    sh /opt/buildserver/setup-env-vars /opt/android-sdk
+    . /etc/profile.d/bsenv.sh
+    fds=/tmp/fds; mkdir -p "$fds"
+    curl -sL https://gitlab.com/fdroid/fdroidserver/-/archive/master/fdroidserver-master.tar.gz \
+      | tar -xz -C "$fds" --strip-components=1
+    export PATH="$fds:$PATH" PYTHONPATH="$fds:$fds/examples"
+    git config --global --add safe.directory "*"
+    # Build the three per-ABI splits exactly as F-Droid does (SOURCE_DATE_EPOCH
+    # comes from THIS merged fdroiddata metadata commit -> matches F-Droid).
+    fdroid build --verbose --no-tarball \
+      org.comunes.tane:$((VC_BASE*10+1)) \
+      org.comunes.tane:$((VC_BASE*10+2)) \
+      org.comunes.tane:$((VC_BASE*10+3))
+    apksigner=$(find "$ANDROID_HOME" -name apksigner -type f | sort -V | tail -1)
+    for f in unsigned/org.comunes.tane_*.apk; do
+      vc=${f##*_}; vc=${vc%.apk}
+      case $((vc%10)) in 1) abi=armeabi-v7a;; 2) abi=arm64-v8a;; 3) abi=x86_64;; esac
+      "$apksigner" sign --ks /tmp/ks.jks --ks-key-alias tane-upload \
+        --ks-pass "env:KSPASS" --key-pass "env:KEYPASS" \
+        --out "/build/app-${abi}-release.apk" "$f"
+      "$apksigner" verify --print-certs "/build/app-${abi}-release.apk" | grep -i SHA-256
+    done
+  '
+```
+
+Set `KSPASS`/`KEYPASS` in the environment before running (never echo them).
+
+Expected: `Signer #1 certificate SHA-256 digest:` =
+`ddfae432091b8248a8a4a1b353487fa626301f4357ef835e94ec312f69418e38`
+(the tane-upload cert = the future `AllowedAPKSigningKeys` value).
+
+Then:
+1. Upload the three `app-<abi>-release.apk` to a git.comunes.org release for the tag.
+2. Add to the recipe: `AllowedAPKSigningKeys: ddfae432…` and per-build
+   `binary: https://git.comunes.org/comunes/tane/releases/download/v%v/app-<abi>-release.apk`.
+3. Open a follow-up MR. F-Droid re-runs `fdroid build`, gets byte-identical output,
+   verifies our signature, and switches to publishing our signed APK.
+4. If `fdroid verify` still differs, keep it F-Droid-signed — no harm.