Add pg_durable compaction adapter

[tjgreen42]

Summary

Add an optional pg_durable adapter for per-index background compaction
requests. The adapter keeps pg_textsearch's core dispatch scheduler-neutral
while providing hardened operator scripts for deployments that use
pg_durable.

Design

• 01_setup_role.sql creates and validates a least-privilege compactor role
  without changing state when required pg_durable capabilities are absent or
  drifted.
• 02_wrapper.sql installs a SECURITY DEFINER request wrapper with a fixed
  search path, explicit target validation, and owner-only helper privileges.
• Each BM25 index is submitted to its own pg_durable loop. pg_textsearch owns
  the per-index identity and stale-target bookkeeping rather than requiring a
  keyed scheduling API upstream.
• Physical identity helpers bind work to the intended database, index OID,
  relfilenode, and creation identity so a stale job cannot compact a
  replacement relation.
• Each loop calls bm25_compact_step() in separate transactions and uses the
  failure policy supplied by pg_durable's seven-argument df.start.
• Fresh-install and upgrade SQL revoke extension-private helper access from
  nonowners using explicitly qualified catalog operators.

The scripts fail closed on hostile or drifted managed objects, authenticate
extension-owned pg_durable entry points, and keep the callback handoff free of
privilege escalation or stale-object reuse.

Merge blocker

Do not merge until microsoft/pg_durable#354 lands in a release. Released
pg_durable v0.2.6 and current main do not provide failure-resilient recurrence.
PR #354 targets v0.2.7 and adds the required on_failure => continue policy
through the seven-argument df.start; this adapter deliberately does not
invent a new df.loop API. The localized capability checks must gain the
released version floor once that release is available.

Test coverage

The static harness verifies wrapper identity, body hashes, extension
ownership, privilege boundaries, callback wiring, and package contents. The
live adapter harness covers transaction callbacks, precise skipping against
v0.2.6 with no side effects, and full execution against the pg_durable #354
implementation.

Stack position

This is the fourth layer of GitHub stack #479 and remains a draft because of
the upstream release blocker. It builds on #477's scheduler-neutral dispatch;
the final layer adds the singleton recovery backstop and operational
packaging.

[codecov]

Codecov Report

❌ Patch coverage is 3.38983% with 57 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/access/compaction.c	3.38%	57 Missing ⚠️

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