feat(network): announce send chunks as an all-or-nothing batch

[andinux]

Problem

/apply coverage on the server is db_version-granular, but a send chunk is not a db_version: one chunk can span several db_versions (no db_version-boundary flush in the chunk builder), and a value larger than max_chunk_size fragments across several chunks within one db_version. So the first chunk touching db_version N marks N covered before N is complete — a later chunk's failure is masked (failures.apply hidden, columns silently missing) and lastOptimisticVersion advances on a partially received db_version.

Fix (client side)

One send_changes call = one all-or-nothing batch:

• Every chunk announces the same global window dbVersionMin = send_checkpoint+1, dbVersionMax = watermark, plus three new fields: batchId (UUIDv7 per call), chunkIndex (0-based), isFinal (true on the last chunk; already computed by the cloudsync_payload_chunks vtab).
• This replaces the per-chunk announce-window tiling (network_announce_min removed) — the fixed window covers db_versions consumed by non-local-change transactions by construction.
• The server (companion change in the cloudsync server repo) advances optimistic only when the final chunk is received, and confirms + appends the whole window only when every chunk of the batch applied without error. A failed batch is never merged: the window reads back as a gap, the failure is surfaced, and the client re-sends it whole under a new batchId. The client checkpoint already follows lastOptimisticVersion backward, so the regress-on-failure signal works unchanged.
• Old clients (monolithic payloads) send no batchId → the server keeps today's per-payload behavior, which is correct because a monolithic payload is a complete db_version set.

Changes

• src/network/network.c: batch UUID per send call; fixed global window announce; batchId/chunkIndex/isFinal in the /apply JSON (both blob and url transports); send-loop SELECT reads is_final instead of db_version_min/max; tiling logic removed.
• src/network/network_private.h: network_announce_min helper removed; network_apply_json_payload exposed for the network unit test.
• test/network_unit.c: announce-tiling test replaced by test_apply_json_payload_batch (fixed window, batch fields, both transports, missing-batch-id rejection).

Compatibility

The current server ignores the new fields and appends the announced window per chunk, so this client can ship ahead of the server change. Response wire format is unchanged (lastOptimisticVersion/lastConfirmedVersion stay integer db_versions).

Testing

• make unittest — all pass (incl. memory-leaks check)
• make network-unittest — all pass (incl. the new batch-payload test)
• make e2e against the live (pre-batch) server — all pass, incl. Send Gap From Clock Hole and all Chunked tests

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