Integrate PostgreSQL 19 features to Citus

[ihalatci]

Umbrella for the work required to make PostgreSQL 19's new features behave correctly under Citus, and to adopt the ones that benefit distributed execution.

This is scoped to feature work. Build, compatibility and CI stabilisation for PG19 are tracked separately in #8597. This feature umbrella is independent and is not a sub-issue of the support umbrella.

Background

PG19's feature set was reviewed against Citus and each feature classified into one of three buckets:

• Just works - the feature is planner- or executor-local and needs no distributed handling.
• Needs Citus modification - the feature changes parse trees, deparse output, planner hooks or storage behaviour in ways Citus must handle explicitly. Everything below is in this bucket.
• Not relevant - single-node-only features with no distributed analogue.

The items below are the "needs Citus modification" set. They split into four groups.

Guardrails: reject or error explicitly rather than silently mishandle

Issue	PR	Local solution / status	Remaining action
#8756	—	Verified implementation plan only: reject REPACK CONCURRENTLY after core-compatible lock lookup; no commit	Approve implementation, add second-session lock regression, review/PR
#8758	—	Verified fail-closed behavior and subtype-specific error/test plan; no commit	Approve implementation, cross-version tests, review/PR
#8761	#8763	Reviewed PG19-gated rejection implemented and PR open	Review and merge

Correctness: distributed behavior fixes

Issue	PR	Local solution / status	Remaining action
#8759	#8766, #8767	Both reviewed fixes published: shared REVOKE ordering and PG19 table grantor propagation	Review/merge both; close issue only after both land
#8760	#8765	Reviewed JSON COPY coordinator-reroute/protocol fix published	Review and merge
#8762	—	Root cause and structural ROW(alias.*)::logical_type plan verified; no commit	Approve implementation across supported ruleutils copies, cross-version tests, review/PR

Adoption and longer-term design

Issue	PR	Local solution / status	Remaining action
#8770	—	Local 3-file implementation tested: INSTRUMENT_IO in ExplainWorkerPlan() plus remote IO option; no published commit	Produce clean commit SHA, release review, approval, PR
#8771	—	No active defect reproduced; 30/30 plan/result probes green; watch item only	Decide close vs retain watch/detection item; no code currently justified
#8772	—	Design completed, implementation deferred; fixed per-node slot ceiling requires admission control	Architecture/design approval before any implementation
#8773	—	AIO adoption measured and deferred; no local implementation	Decide close/defer; future work needs benchmark-backed design; track cache-pollution defect separately

Notes on PR linkage

PRs targeting pg19-support rather than the default branch do not automatically close issues. Associations and remaining actions are recorded explicitly above.