Phase 3: TEST_LOAD_SOURCE (fresh/update/existing) in test/install

[jnasbyupgrade]

Stacked on #28 (phase 2: schema switching). Adds the update+upgrade (U&U) load-mode axis test/install was always meant to carry, per pgxntool/README.asc's documented pattern.

What changed

• `test/install/load.sql` selects fresh/update/existing via the `count_nulls.test_load_mode` GUC (`TEST_LOAD_SOURCE` make var, same propagation mechanism as `TEST_SCHEMA`):
  • fresh (default): unchanged from phases 1/2.
  • update: `CREATE EXTENSION count_nulls VERSION '0.9.6'` then `ALTER EXTENSION UPDATE` - committed, since `test/install` runs outside any per-test rolled-back transaction (the whole reason this pattern needs `test/install` rather than `test/deps.sql` - a real `ALTER EXTENSION UPDATE` has to actually commit to be tested honestly).
  • existing: asserts count_nulls is already installed and at the current version, touches nothing - for a real `pg_upgrade` run external to this invocation (a later phase adds the CI job that drives this).
• New `extension-update-test` CI job: just `make verify-results TEST_LOAD_SOURCE=update`. No external script needed for this leg - unlike a real `pg_upgrade`, an in-place update is pure SQL, so `test/install` can do the whole fresh-vs-updated comparison inside one `pg_regress` invocation.

Deliberately does NOT add a dependency guard yet (the "prove a non-CASCADE drop is blocked" mechanism) - that only earns its keep protecting against an EXTERNAL step corrupting state test/install can't see into (a real `pg_upgrade`), which doesn't exist until the next phase. Adding it here would also immediately conflict with the main suite's own `test__shutdown__drop_all`. Also not crossing this new job with `TEST_SCHEMA` yet - that's its own phase, once the real pg_upgrade job exists too so both can cross schema together.

Verification

Locally against PG17: fresh, update, and existing modes all pass via `make verify-results` - zero expected-output changes needed for any combination, including `update` × `TEST_SCHEMA=Quoted` crossed together, confirming phase 2's schema-invariant design holds across load modes too (load-bearing for the later phase that crosses U&U with schema in CI). `existing` mode verified manually against a real out-of-band `CREATE EXTENSION` + `--use-existing` run.

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