CI: test the declared floor version of build/runtime dependencies (not just PG version)

[jnasbyupgrade]

Summary

Our CI matrix only varies the PostgreSQL version. It never validates that we
actually support the oldest version of a dependency we claim to require
(e.g. META.in.json's cat_tools: 0.3.0 floor) — we only ever build/test
against whatever's current. If the floor drifts out of sync with reality
(newer dependency features get used without bumping the declared floor), CI
won't catch it.

Proposed shape (from prior discussion, not a committed design)

A dependency-version axis, orthogonal to the PG-version matrix, testing each
declared dependency at three points:

1. floor — the oldest version declared in META.in.json's prereqs.
   Blocking. Catches accidental use of dependency features newer than the
   declared floor.
2. released/pinned — whatever's currently used (a real PGXN release, or
   a temporary git-source pin like the one added in Fix cat_tools dependency: install from pinned git SHA, not PGXN #10 for cat_tools
   0.3.0). Blocking. Main day-to-day coverage.
3. master (dependency's development tip) — non-blocking (allow-failure
   or scheduled), early warning for upcoming breakage in the dependency.

To avoid matrix blow-up: run the full PG-version matrix only for the
"released" point; run "floor" and "master" at just the min and max supported
PG versions.

This needs a reusable installer, something like
install-dep <name> <pgxn|git> <version-or-ref> [pg_config], generalized out
of the one-off cat_tools git-pin Makefile target added in #10.

Current blocker

For cat_tools specifically, there's no real "floor" to test against yet:
no tagged/published cat_tools release has
cat_tools.routine__parse_arg_types_text (added on cat_tools' unreleased
master, intended as 0.3.0). The floor case is only testable once that's
tagged.

Status

Not scheduled for implementation. This issue exists so the plan doesn't
get lost and so we remember which project/repo this belongs to
(extension_tools, consumer-side — not cat_tools, which is a leaf with no
dependencies of its own). Revisit once:

• cat_tools 0.3.0 is tagged/published (unblocks the floor case for this repo), and
• the in-flight CI work (no-reinstall test-update GUC mode, pg_tle dependency
  testing) has landed, since this axis interacts with both.

See also advanced-extension-testing.md (§ dependency-version testing axis)
and PR #10.