Add 7 new SQLite unit tests and 7 new PostgreSQL test files covering:
  - DWS/AWS algorithm rejection (unsupported CRDT algos return clean errors)
  - Corrupted payload handling (empty, garbage, truncated, bit-flipped)
  - Payload apply idempotency (3x apply produces identical results)
  - Causal-length tie-breaking determinism (3-way concurrent update convergence)
  - Delete/resurrect with out-of-order payload delivery
  - Large composite primary key (5-column mixed-type PK roundtrip)
  - PostgreSQL-specific type roundtrips (JSONB, TIMESTAMPTZ, NUMERIC, BYTEA)
  - Schema hash mismatch detection (ALTER TABLE without cloudsync workflow)

Add a postgres-build matrix job that compiles the PostgreSQL extension
for 5 platform/arch combinations (linux-x86_64, linux-arm64,
macos-arm64, macos-x86_64, windows-x86_64) and includes them as
release assets. Add postgres-package Makefile target and Windows
platform support (PG_EXTENSION_LIB, -lpostgres linking).

…dows

macOS: install gettext for libintl.h and pass include path via
PG_EXTRA_CFLAGS. Windows: skip POSIX defines (_POSIX_C_SOURCE,
_GNU_SOURCE) so PG headers use Winsock instead of netinet/in.h.

macOS: guard Security.h include behind !CLOUDSYNC_POSTGRESQL_BUILD to
avoid type conflicts (Size, uint64) with PostgreSQL headers, use
getentropy instead. Windows: dropped from postgres-build matrix — MSYS2
PostgreSQL headers expect Unix includes incompatible with MinGW.

Replace _GNU_SOURCE with _DARWIN_C_SOURCE on macOS to expose
preadv/pwritev declarations required by PostgreSQL headers.
_GNU_SOURCE is now Linux-only.

… linking

macOS linker requires explicit handling of undefined symbols. PostgreSQL
extensions resolve symbols at load time, so -undefined dynamic_lookup is
needed. Also consolidate UNAME_S detection to a single location.

macos-13 runners are deprecated. Cross-compile x86_64 on macos-15 ARM
runner via -arch x86_64 passed through PG_EXTRA_CFLAGS. Also pass
PG_EXTRA_CFLAGS to linker for arch flag propagation.