fix: root release evidence paths

[SamuelSupe]

Root cause

The release workflow configured relative report paths such as
artifacts/rollback-drill.json, but Go tests execute with
internal/storage as their working directory. The report was therefore written
under the package directory while actions/upload-artifact searched from the
repository root. The rollback test passed, but the required release evidence
was not uploaded and the final release job could not consume it.

Fix

Normalize relative CAS stress and rollback report paths against the repository
root in scripts/postgres_cas_gate.sh. Absolute paths remain unchanged.

Validation

• bash -n scripts/postgres_cas_gate.sh
• git diff --check
• Formal rollback smoke with
  GRAPHDB_TEST_ROLLBACK_REPORT=artifacts/rollback-fix-smoke.json
• Verified the report exists at repository-root artifacts/ and does not exist
  under internal/storage/artifacts/
• Remote blob 6a741677f5d0e8f35ebc8fb3fe6cb00872686f60 matches the locally validated
  script exactly