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.dockerignore

@@ -0,0 +1,26 @@
+**
+!composer.json
+!composer.lock
+!src/**
+!tests/**
+!db/**
+!config/**
+!templates/**
+!public/index.php
+!bin/**
+!dev.sh
+!dev.ps1
+!phpunit.xml.dist
+!phpcs.xml.dist
+!phinx.php
+!.editorconfig
+!docker/**
+!docker-compose.yml
+!README.md
+!CONTRIBUTING.md
+!LICENSE
+!assets/**
+!package.json
+!package-lock.json
+!.dockerignore
+!.gitignore