test(stack): share one state root across the integration suite

[jgoux]

The stack integration suite gave almost every test its own temporary state root. Automatic public ports skip only ports saved by stacks under the same root, so a concurrently running file could take the port a stopped composition in another file had saved. The restart then failed with Public port … is already in use, as seen intermittently in Supabase.native.integration.test.ts ("promotes a stopped database composition without replacing its identity or data"). An occupied saved port reporting a conflict is the intended product behaviour; the suite was outside the supported topology of one state root per host.

The suite now follows that topology:

• A vitest globalSetup for the integration project creates one state root per run and provides it via inject. Every test that binds real listeners on automatic public ports (including composition creation, which binds immediately) uses it with unique stack IDs.
• Tests that only simulate allocation, bind no public ports, or assert the exact contents of a root (Sweep, State, HostProcess, destroy/rollback, container shutdown and sweep retry, and the malformed-sibling Ports cases) keep private roots.
• In-process owner doubles register through registerLeased, which takes the stack lease before saving, like runStackHost. Without it, another file's orphan sweep could reclaim a stack whose in-process owner holds no lease.
• Whole-root discover and snapshot-descriptor assertions in migrated files check the test's own stacks.
• Native backend ports come from the same span and skip only the claims they are given, so service tests that reserve them read the shared root's claims, and the native reservation tests in Ports.integration.test.ts avoid shared claimed ports while still targeting the port under test.
• makeDockerDatabaseRoot can place a Docker data root under the shared root. The shared root's Docker volume is removed by the existing per-run, label-based cleanup in tests/docker-volume-run.ts.
• Fixed test ports inside the automatic span move below 20000.
• A new Ports regression proves that a competitor whose first scan candidate is a stopped sibling's saved port gets it in a separate root but skips it in the same root.

CommandRunner.native-cleanup.integration.test.ts downloaded psql into a fresh cache inside its test body and timed out on slow downloads; it now uses the shared artifact cache and prepares psql in a beforeAll hook.

Concurrent integration runs from other checkouts on the same host still use separate roots and remain outside the supported topology.