pg_sift 1.3.1: fixes `CREATE INDEX` failing on large tables with a small `maintenance_work_mem`. Install the new files; no `ALTER EXTENSION` step, no `REINDEX` and no on-disk format change.

## Highlights

- **Any table builds at any `maintenance_work_mem`.** A parallel `CREATE INDEX` on a large table failed with `sift parallel build worker needs more than 256 runs` (and a serial one with `would need more than 128 segments`) when the memory budget was small for the table. The build now merges its runs in passes, like an external sort, so the budget only decides how fast it goes (down to 1MB), in parallel or not. When a small budget would need more than 128 segments the build writes fewer, larger ones. The index answers and scores exactly as one built with more memory. Builds at the default or a generous budget take as long as before; at 1MB they take up to 1.5x longer.
- **No workaround needed.** Setting `max_parallel_maintenance_workers = 0` or raising `maintenance_work_mem` is no longer required to build or rebuild an index on a big table.

## Compatibility

- PostgreSQL 17 and 18; PostgreSQL 19 is experimental (CI builds it from git on every push). No on-disk format change; existing indexes keep working.

## Packages

x86-64 packages for PostgreSQL 18 are attached below (`SHA256SUMS` covers every asset); other versions build from source with `make USE_PGXS=1`.

- Debian 13, Ubuntu 24.04 (PGDG `postgresql-18`): `sudo apt install ./postgresql-18-pg-sift_1.3.1-1_amd64.deb`
- RHEL, Rocky, Alma 9 and 10 (PGDG `postgresql18-server`): `sudo dnf install ./pg_sift_18-1.3.1-1.el9.x86_64.rpm`
- Arch Linux: AUR `pg_sift`.

Every change: [CHANGELOG.md](https://github.com/brauliobo/pg_sift/blob/v1.3.1/CHANGELOG.md).

## SQLite extension

pg_sift for SQLite 1.3.1 (`sift`, a loadable extension; a separate component from the PostgreSQL extension above). No change to the extension; it is released with the PostgreSQL one.

- Needs SQLite 3.38 or newer; Linux x86-64, glibc 2.34 or newer. Built on Debian 12, loaded into the SQLite of Debian 12, Ubuntu 24.04 and AlmaLinux 10 before release.
- Storage format 2: nothing to upgrade from 1.3.0.

Download `sift-sqlite-1.3.1-linux-x86_64.tar.gz` (below, covered by `SHA256SUMS` with the packages), unpack it and `.load ./sift` from the `sqlite3`
shell (`sha256sum --ignore-missing -c SHA256SUMS`). Documentation: [sqlite/README.md](https://github.com/brauliobo/pg_sift/blob/v1.3.1/sqlite/README.md).
Every change: [sqlite/CHANGELOG.md](https://github.com/brauliobo/pg_sift/blob/v1.3.1/sqlite/CHANGELOG.md).