Structural comparison in pg-upgrade-test (both twin databases)

[jnasbyupgrade]

Extends bin/compare_fresh_vs_update with an EXISTING_DB argument so it can structurally diff an already-populated database against a fresh install, instead of only ever creating and updating its own scratch database. This lets pg-upgrade-test's twin databases (upgrade_oldest_first/upgrade_current_first, migrated by a real binary pg_upgrade) reuse the same comparison tool the test job's update leg already uses, rather than duplicating that logic.

Since EXISTING_DB was installed independently (its own call to test/helpers/create_test_schema.sql, generating its own random schema), the fresh install this script creates for the comparison discovers and reuses that same schema instead of generating a new one - both sides still need to land in the same schema for the comparison to isolate real divergence rather than a spurious schema-name difference.

pg-upgrade-test's CI job now runs this comparison against both twin databases right after run-suite confirms they're at the current version, catching a divergence class the fixed pgTAP suite doesn't: an object left subtly different (body, comment, ACL) by surviving a real catalog migration, as opposed to only an in-place ALTER EXTENSION UPDATE.

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Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

[jnasbyupgrade]

Folded into #33 - its content (EXISTING_DB support + pg-upgrade-test wiring) now lives on phase6-compare-and-report at 0365bf9, combined with #33's own content and simplified per further review (scratch-vs-scratch mode no longer generates a schema at all).