feat(backend): migrate production backend to Rust

Replace the Go backend with the Rust workspace and route Android/iOS through
UniFFI bindings. Include the migrated extension runtime, network, providers,
media metadata, downloads and library operations, with version 5.0.0+147.

Remove Go sources, adapters, native build selection and CI dependencies.
Build Rust unconditionally and lock the iOS Rust pod using a relative path.
Retain legacy source and migration evidence in a local ignored archive.

Validation: Android Kotlin compile and 53 native tests; Swift Rust-branch
equivalence and syntax; CocoaPods install; workflow, shell, Ruby, plist and
diff checks. Reuse the preceding 100 Rust tests, fmt/Clippy and five-ABI
build checkpoint; no new APK or iOS application build for this cleanup.

Known follow-up: URL/URLSearchParams globals are missing from the Rust JS
runtime. A controlled extension replay confirms a URL-resolution regression;
this commit does not fix that runtime gap.

scripts/check_rust_backend.sh

@@ -0,0 +1,42 @@
+#!/usr/bin/env bash
+# Check the production Rust workspace without local migration archives.
+
+set -euo pipefail
+
+usage() {
+  cat <<'EOF'
+Usage: check_rust_backend.sh
+
+  Run Rust formatting, Clippy, and unit tests.
+  --help    Show this help text.
+EOF
+}
+
+if (( $# > 1 )); then
+  echo "Error: unexpected arguments: $*" >&2
+  usage >&2
+  exit 2
+fi
+if (( $# == 1 )); then
+  case "$1" in
+    --help|-h)
+      usage
+      exit 0
+      ;;
+    *)
+      echo "Error: unknown argument: $1" >&2
+      usage >&2
+      exit 2
+      ;;
+  esac
+fi
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+PROJECT_DIR="$(dirname "$SCRIPT_DIR")"
+
+cd "$PROJECT_DIR/rust_backend"
+# Exercise the same runtime-detected AES backend as the release build.
+export RUSTFLAGS="${RUSTFLAGS:-} --cfg aes_armv8"
+cargo fmt --all -- --check
+cargo clippy --workspace --all-targets --locked -- -D warnings
+cargo test --workspace --locked