ADFA-3771: Document android-sdk.zip image recompression procedure

docs/process/recompress-android-sdk-images.md

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+# Recompressing oversized images in android-sdk.zip
+
+`android-sdk-arm64-v8a.zip` / `android-sdk-armeabi-v7a.zip` are downloaded assets
+(see `debugAssets`/`releaseAssets` in `app/build.gradle.kts`), not built in this
+repo — they're a packaged copy of Android SDK platform data (`platforms/android-<N>/`)
+fetched from appdevforall.org. Some of the bundled platform resources are large,
+unoptimized PNGs that bloat the APK for no benefit (ADFA-3771 found two default
+wallpaper images at ~2.7MB and ~3.9MB).
+
+## Finding an oversized asset
+
+```sh
+unzip -l assets/android-sdk-arm64-v8a.zip | sort -n -k1 | tail -20
+```
+
+## Recompressing
+
+Lossless recompression (`zopflipng`) barely helps here — these PNGs are already
+well DEFLATE-compressed, and re-running DEFLATE on the same pixel data doesn't
+buy much. The real win is `pngquant`'s palette quantization, which is lossy but
+usually visually indistinguishable at these quality settings for
+photographic/gradient content like a wallpaper:
+
+```sh
+pngquant --quality=80-95 --strip --force --output out.png in.png
+```
+
+**`pngquant` on this machine is a snap package** and can't read files outside
+its allowed roots — a path like `/tmp/.../scratchpad/foo.png` fails with
+`cannot open ... for reading` even though the file exists and is readable by
+the same user. Snap confinement only grants read/write access under `$HOME`,
+`/media`, and a few other allowed roots, so copy the input into `$HOME` first
+(the same workaround applies to other snap-packaged CLI tools, e.g. `jira-cli`).
+Confirmed savings for
+ADFA-3771: `drawable-sw600dp-nodpi/default_wallpaper.png` 2.75MB → 844KB (69%),
+`drawable-sw720dp-nodpi/default_wallpaper.png` 3.94MB → 1.43MB (64%).
+
+Always visually diff before/after (open both images) — quantization quality
+varies a lot by image content, and this doesn't have a UI test to catch a bad
+result.
+
+## Updating the zip in place
+
+`zip` can replace a single entry in an existing archive without rewriting the
+whole thing, as long as the working directory mirrors the archive's internal
+path structure. Substitute `android-<N>` below with whichever platform
+directory (from `unzip -l`) actually contains the target asset — it's
+`android-36` for ADFA-3771's SDK level, but will differ for other assets or
+future SDK bumps:
+
+```sh
+mkdir -p work/platforms/android-<N>/data/res/drawable-sw600dp-nodpi
+cp out.png work/platforms/android-<N>/data/res/drawable-sw600dp-nodpi/default_wallpaper.png
+cd work
+zip ../assets/android-sdk-arm64-v8a.zip platforms/android-<N>/data/res/drawable-sw600dp-nodpi/default_wallpaper.png
+```
+
+Repeat for `android-sdk-armeabi-v7a.zip` — SDK platform resources are
+architecture-independent, so both zips carry byte-identical copies of these
+files (verified via `md5sum` before touching either). Verify afterward:
+
+```sh
+unzip -t assets/android-sdk-arm64-v8a.zip     # "No errors detected"
+unzip -t assets/android-sdk-armeabi-v7a.zip   # "No errors detected"
+```
+
+## Scope
+
+This only rewrites the locally downloaded copies under `assets/` (gitignored —
+see `assetsDownloadDebug`/`assetsDownloadRelease` in `app/build.gradle.kts`).
+Getting the recompressed images into the real, published `android-sdk.zip` /
+`android-sdk.zip.br` on appdevforall.org is a separate, external publishing
+step outside this repo.