fix(android): bake android24→26 clang shim into the docker image itself

Build #13's PATH wrapper trick failed because tauri-cli invokes the linker
with an absolute path (/opt/android-sdk/ndk/.../bin/aarch64-linux-android24-
clang), which bypasses \$PATH entirely. The pthread_shim logs confirmed the
broken API-24 stubs were still being linked:

  WZP_pthread_shim: dlsym(RTLD_DEFAULT, pthread_create) returned NULL:
    libdl.a is a stub --- use libdl.so instead

Move the fix up a level — into the Dockerfile itself. On image build, for
each of the four android ABIs × {clang, clang++}, rename
`${abi}24-${suffix}` to `${abi}24-${suffix}.orig` and replace it with a
shell wrapper that exec()s `${abi}26-${suffix}`. Any call to the API-24
wrapper — via PATH, absolute path, or otherwise — now transparently runs
the API-26 wrapper, which uses the real libc.so/libdl.so bindings.

The old bash-c /tmp/wrappers workaround in build-tauri-android.sh is
removed now that the image handles it at the right layer.

Also add `--shell` to build-tauri-android.sh: opens an interactive docker
container on the remote with the same mounts/env as the build, so I can
iterate on cargo tauri android build / manually patch files / etc.
without the full git push → ssh → rebuild → install loop.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/Dockerfile.android-builder

@@ -74,6 +74,27 @@ RUN yes | $ANDROID_HOME/cmdline-tools/latest/bin/sdkmanager --licenses > /dev/nu
         "platform-tools" \
         2>&1 | grep -v '^\[' > /dev/null
 
+# Work around broken API-24 libc/libdl stubs in the NDK. Rust's pre-compiled
+# libstd.rlib for aarch64-linux-android links against pthread_create / dlsym
+# from the sysroot. With --target=...24, those resolve to the API-24 "stub"
+# libc.a/libdl.a that return "libdl.a is a stub --- use libdl.so instead" at
+# runtime. API-26 has real dynamic bindings. Tauri-cli hard-codes the
+# android24-clang path so env var overrides don't stick.
+#
+# Permanent fix inside the image: replace every `${abi}24-clang` binary with
+# a shim that exec()s the corresponding `${abi}26-clang`. Any build using
+# this image now transparently gets the API-26 sysroot even when the caller
+# asks for API-24.
+RUN set -eux; \
+    BIN=$ANDROID_NDK_HOME/toolchains/llvm/prebuilt/linux-x86_64/bin; \
+    for abi in aarch64-linux-android armv7a-linux-androideabi i686-linux-android x86_64-linux-android; do \
+        for suffix in clang clang++; do \
+            mv "$BIN/${abi}24-${suffix}" "$BIN/${abi}24-${suffix}.orig"; \
+            printf '#!/bin/sh\nexec "%s/%s26-%s" "$@"\n' "$BIN" "$abi" "$suffix" > "$BIN/${abi}24-${suffix}"; \
+            chmod +x "$BIN/${abi}24-${suffix}"; \
+        done; \
+    done
+
 # Make SDK world-readable so builder user can access it
 RUN chmod -R a+rX $ANDROID_HOME
 

scripts/build-tauri-android.sh

@@ -36,6 +36,7 @@ REBUILD_RUST=0
 DO_PULL=1
 DO_INIT=0
 BUILD_RELEASE=0
+DROP_SHELL=0
 for arg in "$@"; do
     case "$arg" in
         --rust)     REBUILD_RUST=1 ;;
@@ -43,6 +44,7 @@ for arg in "$@"; do
         --no-pull)  DO_PULL=0 ;;
         --init)     DO_INIT=1 ;;
         --release)  BUILD_RELEASE=1 ;;
+        --shell)    DROP_SHELL=1 ;;    # interactive debug shell inside container
         -h|--help)
             sed -n '3,30p' "$0"
             exit 0
@@ -50,6 +52,34 @@ for arg in "$@"; do
     esac
 done
 
+# ── --shell: drop into an interactive container for fast manual iteration ──
+# The container is NOT --rm'd so you can keep hacking between invocations,
+# and has the same mounts / env as the build path above so `cargo tauri
+# android build ...` just works.
+if [ "$DROP_SHELL" = "1" ]; then
+    log "Starting interactive shell in wzp-android-builder container..."
+    log "  cd /build/source/desktop/src-tauri && cargo tauri android build --debug --target aarch64 --apk"
+    log "  (exit the shell with ^D; container will be removed)"
+    ssh -t -A $SSH_OPTS "$REMOTE_HOST" "
+        set -euo pipefail
+        BASE=$BASE_DIR
+        # Make sure the source/cache is writable by uid 1000
+        sudo chown -R 1000:1000 \$BASE/data/source \$BASE/data/cache 2>/dev/null || true
+        docker run --rm -it \
+            --user 1000:1000 \
+            -v \$BASE/data/source:/build/source \
+            -v \$BASE/data/cache/cargo-registry:/home/builder/.cargo/registry \
+            -v \$BASE/data/cache/cargo-git:/home/builder/.cargo/git \
+            -v \$BASE/data/cache/target:/build/source/target \
+            -v \$BASE/data/cache/gradle:/home/builder/.gradle \
+            -v \$BASE/data/cache/android-home:/home/builder/.android \
+            -w /build/source/desktop/src-tauri \
+            wzp-android-builder \
+            bash
+    "
+    exit 0
+fi
+
 log() { echo -e "\033[1;36m>>> $*\033[0m"; }
 ssh_cmd() { ssh -A $SSH_OPTS "$REMOTE_HOST" "$@"; }
 
@@ -166,30 +196,6 @@ docker run --rm \
     wzp-android-builder \
     bash -c '
 set -euo pipefail
-
-# ─── Linker wrappers ───────────────────────────────────────────────────────
-# Tauri-cli hard-codes aarch64-linux-android24-clang as the Rust linker for
-# the aarch64 target, which resolves pthread_create / dlsym / __init_tcb
-# against the NDK API-24 *stub* libc.a/libdl.a. Those stubs are designed to
-# crash if called — they only exist so the API-24 sysroot compiles, not so
-# symbols actually work. API-26 has the real dynamic bindings to libc.so.
-#
-# Rather than fight tauri-cli to change which clang it invokes, we put a
-# wrapper on $PATH that IS named android24-clang but exec()s the android26
-# version. Same trick works for every ABI.
-mkdir -p /tmp/wrappers
-for abi in aarch64-linux-android armv7a-linux-androideabi i686-linux-android x86_64-linux-android; do
-    for suffix in clang clang++; do
-        cat > /tmp/wrappers/${abi}24-${suffix} <<WRAPPER
-#!/bin/sh
-exec /opt/android-sdk/ndk/26.1.10909125/toolchains/llvm/prebuilt/linux-x86_64/bin/${abi}26-${suffix} "\$@"
-WRAPPER
-        chmod +x /tmp/wrappers/${abi}24-${suffix}
-    done
-done
-export PATH=/tmp/wrappers:$PATH
-echo ">>> installed android24→android26 linker wrappers in /tmp/wrappers"
-
 cd /build/source/desktop
 
 echo ">>> npm install"