Fix disposable PostgreSQL startup race in CI #28

The development push run 39517411309 fails during disposable PostgreSQL setup. The Unix-socket readiness check can connect to the image's temporary initialization server, which then shuts down before schema setup.

Scope: wait for the final PostgreSQL TCP listener, use that same transport for test queries, expose SQL diagnostics, and reproduce the race with delayed initialization in the isolated fixture. Production collector behavior is unchanged.

Acceptance: the delayed fixture fails with the old readiness check, passes after the fix, and all repository checks pass on the PR and merged development commit.

Follow-up to #20 (TASK-OBS-05). Requirement: PapyrusReader/docs#80 (NFR-9_2).

Verified: the delayed initialization reproduced premature readiness before the fix. All 14 tests and all native, WireGuard, and stack smoke checks passed on PR #29 and on merged development commit c2aaf28c09f866f0037199869ae93d7c19bcdf6e.

Post-merge CI: https://github.com/PapyrusReader/observability/actions/runs/39519115002