test: validate M3 imports and locking on PostgreSQL

Add isolated PostgreSQL fixtures, two-session locking coverage, and a disposable remote Docker test runner. Correct the canonical-root fixture and document verified SQLite and PostgreSQL results.

docker/php/Dockerfile.test

@@ -0,0 +1,11 @@
+FROM php:8.4-cli-bookworm
+
+RUN apt-get update && apt-get install -y --no-install-recommends libonig-dev libxml2-dev libpq-dev libsqlite3-dev unzip \
+    && docker-php-ext-install mbstring dom pdo_pgsql pdo_sqlite xmlwriter \
+    && rm -rf /var/lib/apt/lists/*
+COPY --from=composer:2 /usr/bin/composer /usr/local/bin/composer
+WORKDIR /workspace
+COPY composer.json composer.lock ./
+RUN composer install --no-interaction --prefer-dist --no-progress
+COPY . .
+CMD ["composer", "test"]