build(android): carry the Wails Android scaffolding verbatim

Plan 015 phase 0 established that this app cross-compiles for Android
with no source changes at all. A CGO_ENABLED=0 probe of the whole tree
for android/arm64 fails on exactly two packages -- ebitengine/oto/v3
and wails/v3/pkg/application -- and both fail only because their
Android implementation is cgo, which is what the NDK supplies. Notably
modernc.org/sqlite, the entire database layer and the thing most likely
to have no Android target, is clean. The fat APK (arm64-v8a + x86_64)
builds in about 25 seconds.

So build/android/ stops being ignored. This commit is the tree exactly
as `wails3 generate build-assets` emits it, so that the next commit is
a readable diff of what we changed and a future refresh has something
to compare against.

Two things about how it is carried:

`wails3 update build-assets` does NOT generate it, contrary to what
CLAUDE.md has claimed since the v3 migration. In beta.8 that command
extracts only internal/commands/updatable_build_assets, which is
darwin/ios/linux/windows; the android tree comes from `generate
build-assets`, which rewrites the whole of build/. It was generated
once into a scratch directory and copied across, so from here it is
committed and hand-edited like source. Only its output is ignored --
jniLibs (~60MB of per-ABI c-shared libraries), gen/, overlay.json and
Gradle's own directories.

And it brings one Go file into ./... -- scripts/deps/install_deps.go,
the interactive SDK installer behind `task android:install:deps`, which
trips 24 of our strict linters. golangci excludes the directory rather
than reformatting upstream's file, which the next refresh would undo
and which would make the diff against upstream unreadable. This repo
uses `make android-setup` instead.

.gitignore

@@ -63,10 +63,24 @@ bin/
 # packaging tasks that depend on it. A derived file with one source.
 build/linux/yellowjacket.desktop
 
-# `wails3 task common:update:build-assets` regenerates the mobile trees
-# whether or not anything asks for them. This is a desktop player and
-# cannot target iOS/Android, so their includes: entries are dropped from
-# Taskfile.yml and the trees themselves are not carried — ignored rather
-# than deleted-and-rediscovered on every asset refresh.
+# iOS is not carried. `wails3 update build-assets` regenerates the tree
+# whether or not anything asks for it, so it is ignored rather than
+# deleted-and-rediscovered on every asset refresh, and its includes:
+# entry is dropped from Taskfile.yml.
+#
+# build/android/ *is* carried — see plan 015. Note that `update
+# build-assets` does NOT regenerate it (only `generate build-assets`
+# does, and that rewrites the whole of build/), so the tree is committed
+# and edited by hand like any other source. Only its output is ignored,
+# below.
 build/ios/
-build/android/
+
+# Android build output. jniLibs holds the ~30 MB per-ABI c-shared
+# libraries the Go build produces; gen/ and overlay.json are written by
+# `wails3 android overlay:gen`; the rest is Gradle's.
+build/android/app/src/main/jniLibs/
+build/android/app/build/
+build/android/build/
+build/android/.gradle/
+build/android/gen/
+build/android/overlay.json