doc: mark the materialize-sink-id header and HEADERS option as GA - #39829

#39830)

The Headers section of the Kafka sink page was marked private preview as
a whole, including the always-on `materialize-timestamp` header. This
scopes the private preview callouts to the two features that are
actually in private preview.

- Replaces the section-level private preview callout with a table of the
headers a Kafka sink emits (`materialize-timestamp`,
`materialize-sink-id`, and user-defined headers from the `HEADERS`
option) and what each contains.
- Moves `materialize-sink-id` into its own subsection, still marked
private preview, with a note that it becomes GA in v26.47.
- Moves the `HEADERS` option into its own subsection, still marked
private preview, with a note that it becomes GA in v26.47.
- Expands the `HEADERS` known limitations: duplicate keys can't be
expressed, `materialize-` keys are silently dropped, the headers column
also appears in the message body, and the option works only with the
upsert envelope.

Part of [SS-504](https://linear.app/materializeinc/issue/SS-504). #39829
(staged, merge next week) removes both private preview callouts when
they go GA in v26.47.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 5.5 <[email protected]>

…ader (#40120)

Adds a new **Export data > Patterns** section with one page,
**Transactional downstream updates using Kafka**. The page shows how a
consumer loading a Kafka sink into another store (Snowflake example) can
expose only timestamps the sink has fully committed, by gating on the
sink's progress frontier and joining data and progress records via the
`materialize-sink-id` header.

### Where each fact comes from

| Fact | Source |
|------|--------|
| Every message carries `materialize-timestamp` (value is the timestamp
as a decimal string) | `src/storage/src/sink/kafka.rs`
(`KafkaTxProducer::send`, `time.to_string()`) |
| `materialize-sink-id` header value is the sink's `GlobalId` (for
example `u123`), emitted when `kafka_sink_emit_sink_id_header` is on |
`src/storage/src/sink/kafka.rs` (`sink_id_header: ...
sink_id.to_string()`); `src/storage-types/src/dyncfgs.rs` |
| Progress record key is `mz-sink-{sink_id}` |
`src/storage-client/src/sink.rs` (`ProgressKey::new`) |
| Progress record value is JSON `{"frontier":[<ts>],"version":<n>}`;
`frontier` is the exclusive upper of committed times |
`src/storage/src/sink/kafka.rs` (`ProgressRecord`, `serialize_frontier`,
`commit_transaction`, `progress_record_migration` test) |
| Progress record is written in the same Kafka transaction as the data |
`src/storage/src/sink/kafka.rs` (`commit_transaction`) |
| A transaction and progress record are committed on every frontier
advance, even with no data | `src/storage/src/sink/kafka.rs` (sink loop,
`Event::Progress` branch) |
| Default progress topic name
`_materialize-progress-{env}-{connection}`, shared by all sinks on a
connection; name exposed in `mz_kafka_connections.sink_progress_topic` |
`src/storage-types/src/connections.rs` (`progress_topic`);
`doc/user/content/sql/create-sink/kafka.md`;
`doc/user/content/sql/system-catalog/mz_catalog.md` |
| Consumers need `isolation.level=read_committed`; exactly-once details
| `doc/user/content/sql/create-sink/kafka.md` (Exactly-once processing)
|
| `materialize-` header prefix is reserved |
`src/storage/src/sink/kafka.rs`;
`doc/user/content/sql/create-sink/kafka.md` |
| Snowflake connector stores payload in `RECORD_CONTENT`, headers in
`RECORD_METADATA:headers`, and the key in `RECORD_METADATA:key` only
with `StringConverter` | [Snowflake Kafka connector
overview](https://docs.snowflake.com/en/user-guide/kafka-connector-overview)
|

### Notes for reviewers

- **GA status:** the `materialize-sink-id` header and the `HEADERS`
option are GA in v26.48 (#39908 turns both flags on by default), so the
page has no private preview caveat. The Kafka sink docs drop their
preview callouts in #39829, and the v26.48.0 release notes (#40113) link
to this page.
- The Snowflake SQL is marked illustrative. The page names no connector
settings except `key.converter`, which the Snowflake docs confirm.

Part of [SS-504](https://linear.app/materializeinc/issue/SS-504).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 5.5 <[email protected]>