Support PostgreSQL 14-18, add regression suite and CI

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Summary

Brings the module from its last compatibility target (PostgreSQL 13) up to the full set of upstream-supported majors, and adds a CI pipeline.

Code (plan_filter.c)

• Requires PostgreSQL 14+ (compile-time #error below that); the pre-13 planner-hook compatibility macros are gone since the 13+ hook signature is universal in the supported window
• _PG_fini removed — PostgreSQL 15 removed library unloading, and it was unreachable before that
• The plan_filter. GUC prefix is reserved (MarkGUCPrefixReserved on 15+, EmitWarningsOnPlaceholders on 14), so a misspelled setting now errors instead of silently leaving the filter off
• PG_MODULE_MAGIC_EXT with module name/version on 18+, plain magic earlier
• Fixed a typo in the error hint

Tests

• The manual testlimit.sql (100M-row table, no pass/fail) is replaced by a pg_regress suite run via make installcheck
• All assertions are behavioral and no cost numbers appear in the output, so a single expected file serves every supported major on every platform
• Covers: limit off/exceeded, cheap statements passing, EXPLAIN being blocked, the SET LOCAL escape hatch and its expiry, filter_select_only both ways, and SUSET enforcement of both GUCs
• The suite pins both GUCs right after LOAD, so it also passes against clusters that preload the module with a limit configured

CI (.github/workflows/ci.yml)

• Gating: build with -Werror + installcheck on PostgreSQL 14, 15, 16, 17, and 18 (pgdg packages, self-owned throwaway cluster per job); cppcheck static analysis
• Report-only: pgbench overhead benchmark (module absent vs. loaded with an unreached limit), published to the job summary and kept as an artifact; continue-on-error because shared-runner numbers are too noisy to gate on
• GITHUB_TOKEN restricted to contents: read

Docs

• README: fixed filter_select_only being documented under a nonexistent name (limit_select_only), documented the supported-version policy and testing, removed pre-9.2 material, added the CI badge
• Decision records in docs/adr/ (support window, test design, CI design) and a new ARCHITECTURE.md

Testing

• Build is warning-free and installcheck passes on PostgreSQL 14-18 in an ubuntu:24.04 rehearsal of the exact CI steps — each version tested against both a pristine cluster and one preconfigured with session_preload_libraries and a cost limit — and on PostgreSQL 18.3 on macOS
• The cppcheck invocation and the benchmark harness were verified in the same rehearsal (bench also on macOS, leaving no temp directories behind)