Phase 4: real pg_upgrade support via a reduced bin/test_existing

[jnasbyupgrade]

Stacked on #30 (phase 3.5: CI hygiene). Adds binary `pg_upgrade` coverage - the one U&U scenario that genuinely can't live inside a single `pg_regress` invocation, since `pg_upgrade` is an external binary process.

What changed

• New `pg-upgrade-test` CI job: install 0.9.6 on an old PostgreSQL major, plant + prove the dependency guard, binary `pg_upgrade` to a newer major, `ALTER EXTENSION UPDATE` the migrated objects, then run the suite against the real upgraded database in existing mode. Matrix: 10→18 and 12→18 (no bridge-update leg - count_nulls has always been pure SQL functions with no `SELECT *`-over-catalog views, so it has no known pg_upgrade-unsafe old version to bridge past). No stepwise every-major climb either, for the same reason (nothing catalog-touching to break at a specific boundary).
• `bin/test_existing`: notably smaller than it would have needed to be before phase 3's `test/install` work. Only `prepare-old` (old-cluster setup before the real `pg_upgrade` binary runs) and `run-suite` (assert + re-prove the guard + run the suite via `--use-existing` afterward) are genuinely external-to-`pg_regress` concerns, plus a small `update` subcommand for the post-upgrade `ALTER EXTENSION UPDATE` step. There's no `update-scenario` subcommand at all - that whole scenario is just `make test-update` now (phase 3's `test/install`-driven update mode), since an in-place update has no external step to drive.
• `run_suite()` gates on plain `make test`, not the old belt-and-suspenders `make test && make verify-results` combo - pgxntool 2.3.0 (Update pgxntool to 2.3.0 #25) already made `make test` itself exit non-zero on regression failures, so the extra call is redundant now.

Not yet crossed with `TEST_SCHEMA` - that's the next phase, once both this job and `extension-update-test` can cross it together.

Verification

Locally against PG17: `prepare-old` → `update` → `run-suite` all pass end to end, without a real `pg_upgrade` (this container's PG12/PG17 clusters are persistent shared dev infra, not disposable, so - same reasoning as the earlier pg-tle-test work - the actual binary `pg_upgrade` leg is left for CI's ephemeral containers to exercise).

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