publish.sh: verify credentials and upload the index last; add next-version-code.sh

Closes #5
Closes #6

An F-Droid repo is a signed index plus the files it names, and a partial
upload could previously leave them disagreeing: aws s3 sync walks the tree
in its own order, so an interrupted publish could land the index before the
APKs it names, breaking the repo for every client until the upload finished
or was rolled back. publish.sh now uploads in two passes, unconditionally --
every non-index file first, the index (index-v1.json(.asc), index-v2.json,
entry.json, entry.jar) last -- so an interruption anywhere in pass 1 simply
leaves the repo describing the old state a little longer, never a mixture.
Credentials are also verified with one real authenticated call
(head-bucket) before either pass, rather than merely checking the
environment variables are non-empty, since an unusable-but-present
credential failing halfway through is exactly how a partial upload happens.

next-version-code.sh is the one implementation of the versionCode rule
(candidate = minutes since 2026-01-01Z, clamped to strictly exceed whatever
is already published for that applicationId), shared by CI and manual
builds so there is exactly one place this can drift. bit pointed out
(appended to #6) that the clamp is monotonic against *observed* state, not
monotonic by construction: two jobs starting in the same minute can both
read the same published value and race to publish the same versionCode on
different APKs. Nothing can prevent that wasted build, so publish.sh now
also refuses to overwrite an already-existing APK object before pass 1 --
turning the collision from a silent, permanent wrong state into a loud
failure at the last moment the information is reliable.

Tested offline (83 assertions: pure-stdlib suites for both scripts, the
real CLIs through with-nixpkgs.sh, and a small local HEAD-only S3 stub
proving the credential check is a real authenticated call and the
APK-collision refusal fires only when it should) and live against
fdroid-weblib-dev (7 more: a real publish observed mid-flight via a
same-process synchronization hook to prove the index is genuinely absent
while the file is already present, both objects reading back
byte-identical, and a second publish refused against a real
already-published APK with its bytes left unchanged).

Co-authored-by: bit <bit@das-labor.org>

action.yml

@@ -1,5 +1,5 @@
-# Makes sign-apk.sh, fdroid-update.sh, publish.sh and with-nixpkgs.sh
-# available to a workflow and reports where they are.
+# Makes sign-apk.sh, fdroid-update.sh, publish.sh, next-version-code.sh and
+# with-nixpkgs.sh available to a workflow and reports where they are.
 #
 # Consume it with the **full URL**, the same reason recorded in weblib-ci's
 # README: `uses: owner/repo@main` resolves against the instance's default
@@ -17,15 +17,17 @@
 name: F-Droid repo publishing tools
 description: >-
   Shared, key-free tooling to align-check and sign a prebuilt APK, run
-  `fdroid update` over a binary repo, and publish the result to an
-  S3-compatible bucket. The caller supplies the keys, the bucket and the
-  APK; this action supplies the pinned toolchain and the scripts.
+  `fdroid update` over a binary repo, compute the next monotonic versionCode,
+  and publish the result to an S3-compatible bucket. The caller supplies the
+  keys, the bucket and the APK; this action supplies the pinned toolchain and
+  the scripts.
 
 outputs:
   path:
     description: >-
       Directory holding with-nixpkgs.sh, nixpkgs-pin, sign-apk.sh,
-      fdroid-update.sh and publish.sh.
+      fdroid-update.sh, next-version-code.sh, next_version_code.py and
+      publish.sh.
     value: ${{ github.action_path }}
 
 runs:
@@ -34,6 +36,7 @@ runs:
     - shell: bash
       run: |
         echo "fdroid-repo-ci tools at ${{ github.action_path }}"
-        for f in with-nixpkgs.sh nixpkgs-pin sign-apk.sh fdroid-update.sh publish.sh; do
+        for f in with-nixpkgs.sh nixpkgs-pin sign-apk.sh fdroid-update.sh \
+                 next-version-code.sh next_version_code.py publish.sh; do
           test -f "${{ github.action_path }}/$f"
         done