Add chunked payload generation and network transport

[marcobambini]

Summary

This PR adds chunk-aware payload generation and send-path transport support to sqlite-sync. It keeps the existing monolithic payload APIs intact while adding a streaming path for large rowsets and oversized individual BLOB/TEXT values.

Implemented changes:

• Adds cloudsync_payload_chunks():
  • SQLite virtual table interface.
  • PostgreSQL set-returning function interface.
  • Three optional inputs: since_db_version, filter_site_id / local site id, and until_db_version.
  • Emits per-chunk metadata including chunk index, payload size, row count, db version range, and stable watermark.
• Adds global persisted setting payload_max_chunk_size:
  • Default: 5 MB.
  • Technical minimum: 256 KB, with lower values clamped.
  • Controls only payload generation, not apply-time acceptance.
• Adds v3 fragment payload support for oversized single BLOB/TEXT values:
  • Large values can be split across multiple transport chunks transparently.
  • Receivers stage fragments internally and apply the final value once all fragments arrive.
  • Duplicate fragment delivery is idempotent and stale incomplete fragment groups are cleaned up.
• Keeps backward compatibility:
  • Existing cloudsync_payload_encode() remains supported for monolithic payloads.
  • cloudsync_payload_apply() accepts legacy payloads, monolithic payloads, and v3 fragment payloads regardless of the local chunk-size setting.
• Updates cloudsync_network_send_changes():
  • Streams outgoing changes from cloudsync_payload_chunks() instead of building one large payload first.
  • Sends each chunk through the existing /apply backend contract, either inline as blob or through upload url.
  • Advances the local send checkpoint only after the chunk stream completes successfully.
  • Merges remote status responses monotonically while multiple chunks are in flight.
• Updates PostgreSQL Docker debug images to build/install dblink, required by the test suite.
• Updates API, performance docs, README, and changelog.
• Adds extensive SQLite unit coverage and PostgreSQL SQL coverage for chunk sizing, large-value fragmentation, out-of-order fragments, stale fragment cleanup, and legacy monolithic apply compatibility.

Compatibility

Existing users of cloudsync_payload_encode() and current network APIs continue to work. The new chunking behavior is opt-in for direct SQL callers via cloudsync_payload_chunks(), and automatic for the built-in network send path. Incoming payload apply remains format-compatible with older payloads and does not reject payloads based on the local payload_max_chunk_size.

Companion backend PR

• cloudsync: https://github.com/sqliteai/cloudsync/pull/45

Testing

• make
• make unittest
• make test reached and passed the SQLite/unit portion, then stopped at the remote e2e stage because INTEGRATION_TEST_DATABASE_ID is not set locally.
• make postgres-docker-debug-rebuild
• Full PostgreSQL suite via Docker container: psql -U postgres -d postgres -f test/postgresql/full_test.sql (Failures: 0). This was run inside the debug container because the debug PostgreSQL build listens on loopback inside the container and the host make postgres-docker-run-test connection is closed by that setup.
• git diff --check

[andinux]

Update — commit 92a048c

Adds the /check download-path support to cloudsync_payload_chunks plus related fixes.

Changes

• exclude_filter_site_id flag on cloudsync_payload_chunks (SQLite hidden column / PG 4th arg): stream all sites except filter_site_id; error if set without a site_id.
• New cloudsync_uuid_text() / cloudsync_uuid_blob() (SQLite + PG) to convert site_id between 16-byte binary and canonical UUID string, so string callers (the /check endpoint) can pass a site_id.
• perf: throttled the v3 fragment stale-GC to once/60s per connection — removes an O(n²) full-table scan that ran on every applied fragment.
• best_index rewritten to assign argv in canonical column order (latent arg-ordering bug fix).
• PostgreSQL 1.0→1.1 migration for the new SQL surface.
• build: hermetic env for curl ./configure (CURL_CONFIG_ENV).
• tests: renamed 39_payload_chunks.sql → 52 (39 was duplicated); added multi-site exclude, UUID roundtrip, and stale-GC-throttle coverage.

Verified: SQLite unittest green; PG full_test.sql → Failures: 0.

TODO

• Cross-engine v3 fragment round-trip test (SQLite ↔ PG) to lock the payload interop invariant.
• Integration tests on CI: network send path + receive split-delivery (needs INTEGRATION_TEST_DATABASE_ID).
• Backend (sqliteai/cloudsync#45): make /check download client-capability-aware — monolithic/v2 for ≤1.0.x clients, chunked/v3 only for new ones; adopt the generate-once download model.
• Minor cleanups: strtoll(...,10) in dbutils_settings_get_value; contract comment on the memset-from-eof vtab reset; named constant for the fragment-sizing iteration count.