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.

docs/postgres-tests.md

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+# PostgreSQL integration tests
+
+From a checkout on a Linux Docker host, run:
+
+```sh
+sh bin/test-postgres.sh
+```
+
+This builds `docker/php/Dockerfile.test` from the current checkout and lockfile,
+starts a disposable PostgreSQL 17 container on a unique network, runs the full
+suite with PostgreSQL enabled, then runs lint. It publishes no ports, mounts no
+existing database volumes, and removes its database container, network and test
+image on exit. Build layers remain cached. PHP 8.4 is used for the PHPUnit 12 test
+runtime; the production image is unchanged. Docker and network access for the
+initial image/dependency build are required.
+
+For remote validation, copy the current source to a **separate** remote directory
+(do not overwrite an existing dirty checkout), then execute the command there.
+Do not copy `.env`, host `vendor`, collection directories or other secrets.
+Capture the command exit status along with its output.
+
+## Existing dedicated test database
+
+Alternatively, export these variables before invoking `composer test`:
+
+```sh
+export MESHVAULT_TEST_PG_DSN='pgsql:host=127.0.0.1;port=5432;dbname=meshvault_test'
+export MESHVAULT_TEST_PG_USER=meshvault_test
+# Supply MESHVAULT_TEST_PG_PASSWORD securely through your environment.
+composer test
+```
+
+Use a dedicated test database and a role permitted to create schemas. Each test
+migrates production Phinx migrations into a randomly named schema, sets its
+connections' search path to that schema only, and drops that schema on teardown.
+Configured connection or migration errors fail the suite rather than skip it.
+An abruptly killed process may leave `meshvault_test_*` schemas; only remove
+schemas known to belong to a finished test run.
+
+Without `MESHVAULT_TEST_PG_DSN`, repository and pipeline tests continue to use
+SQLite and the five PostgreSQL-only tests skip. Those tests cover schema isolation,
+overlapping claims using `SKIP LOCKED`, rollback/reclaim, and item-lock timeout and
+retry using two independent PDO sessions. They are not a multiprocess load test.
+
+The older `MigrationTest` suite uses the separate application `MESHVAULT_DB_*`
+variables and remains skipped in this isolated runner. Production migrations are
+nevertheless exercised by the repository/pipeline fixtures on PostgreSQL.