chore: add release reproducibility config

[ovitrif]

Closes #953
Related: #1006
Replaces the release reproducibility portion of #959.

Description

This PR splits the reproducibility work out of #959:

• Adds the manual Reproducible Release workflow for mainnetRelease reproduction artifacts.
• Adds scripts/reproduce-release.sh to build/reuse the release AAB, recreate APK splits with pinned bundletool, extract arm64 native libraries, and write checksums.
• Documents the local and GitHub Actions reproduction flow in docs/reproducible-builds.md.
• Documents the remaining upstream native-library reproducibility work related to v2.2.0: Native libraries not reproducible (libbitkitcore.so, libdatastore_shared_counter.so, libjnidispatch.so, libpubky_app_specs) #953.

Preview

N/A

QA Notes

Manual workflow behavior should be verified in GitHub Actions (after merge).

Validated locally:

bash -n scripts/reproduce-release.sh
go run github.com/rhysd/actionlint/cmd/actionlint@latest .github/workflows/reproducible-release.yml
git diff --check
tmpdir=$(mktemp -d); jar="$tmpdir/bundletool.jar"; curl --fail --location --silent --show-error https://github.com/google/bundletool/releases/download/1.18.1/bundletool-all-1.18.1.jar --output "$jar"; shasum -a 256 "$jar"; java -jar "$jar" version; rm -rf "$tmpdir"