SS-489 - storage: let MySQL exports progress independently during a snapshot - #40207

…#40206)

### Motivation

Today a MySQL source snapshots all of its tables before it reads any of
the binlog. This PR lets the replication operator read the binlog while
the snapshot runs, as #39419 did for PostgreSQL. #40207 then lets each
export's frontier advance on its own.

NOTE: this needs an exception added for the 2s increase in snapshot
performance, but I can't add it until I have a merge hash from main.
Will do in a follow up PR.

Closes: [SS-488](https://linear.app/materializeinc/issue/SS-488)

### Description

All new behavior is keyed off
`storage_persist_sink_description_lookahead` being nonzero. This is the
same switch PostgreSQL uses.

With a nonzero lookahead, the snapshot operator emits its rewind
requests as soon as the snapshot GTID frontier is known. That happens
before any table is read. The replication operator then reads the binlog
while the tables are read. Every export still advances together.
Replication data is staged in the dataflow until the snapshot completes.

With the lookahead at zero, rewinds are emitted after the snapshot
completes, as before.

The snapshot operator reads the setting once at render time. That
matches the value the persist sink reads in the same dataflow build.

MySQL sources now count as OLTP sources in `render.rs`. A snapshotting
MySQL export therefore gets the persist sink's snapshot ceiling and
lookahead grouping, like a PostgreSQL export. The mzcompose default sets
the lookahead to 2s. The MySQL initial load feature benchmarks should
see their first rows about that much later, as the PgCdc benchmarks did
after #39602.

#39419 also added a leader check that the replication slot exists before
the snapshot starts. MySQL needs no counterpart. The replication
operator already checks `@@gtid_purged` against the resume upper before
it waits for rewind requests.

### Verification

The mzcompose default sets the lookahead, so the existing
mzcompose-based MySQL source tests exercise the early rewind path.
#40208 adds targeted coverage.

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

---------

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

Dates in RC snapshots are provisional until the release ships. One
commit is added per snapshot; the final snapshot sets the real dates.
**The `final` snapshot has been applied: the dates below are the real
ones** (Cloud 2026-11-12, Self-Managed 2026-11-13).

The `final` snapshot adds no entries — `v26.50.0-rc.1...v26.50.0` is one
commit, the version bump, and no agent-skills PR merged in the window.
Its substantive work was the dates, a rebase onto `main`, and the three
removals below.

**Rebased onto `main`.** The branch was 107 commits behind and
conflicted: `main`'s `## v26.49.1` section replaced the stale `##
v26.49.0` stub the branch was cut from. Resolved by keeping `main`'s
section verbatim and re-inserting `## v26.50.0` above it; same for the
compatibility YAML (`v26.50`, then `v26.49.1`, then `v26.48.2`).

**Three entries removed as already published on `main`.** The `v26.49.1`
patch train cherry-picked four Console PRs that also merged to `main`
inside this train's `rc.1` window, so both trains classified them
independently and `main` already carries them under `## v26.49.1`:

| PR(s) | Published on `main` in `## v26.49.1` as | Removed from `##
v26.50.0` |
|---|---|---|
| [#40044](#40044),
[#40074](#40074) | the
feature section *"Right size your clusters, using the new Hydration tab
on the console"*, with a screenshot | *"**Diagnose hydration from the
cluster details page**"* |
| [#40511](#40511) |
*"**Freshness and cluster details pages on Self-Managed**"*, covering
both pages | *"**The Freshness page on Self-Managed**"* |
| [#40492](#40492) |
the Bug Fix about the Console's clusters list showing a dash in the Disk
column | *"**Swap usage in the Console's Disk column**"* |

`main`'s wording is the later and fuller treatment in all three cases.
That leaves `## v26.50.0` with **5 Improvements and 7 Bug Fixes**.

Decisions required:

- **MCP protocol default flip**
([#39762](#39762)):
published as an Improvement because `enable_mcp_protocol_2026_07_28` is
`true` at the tag and the flip *is* the change. The PR was held in draft
"until the 2026-07-28 code ships in a release and passes a test on a
deployed environment" — is it confirmed on?
- **Columnar accumulators**
([#40364](#40364)):
omitted as Borderline — the default is `true` but the PR states no
measured user-facing effect. Is there a memory or CPU figure worth
publishing?
- **MySQL snapshot/replication concurrency**
([#40206](#40206),
[#40207](#40207)):
omitted as Borderline — gated on
`storage_persist_sink_description_lookahead`, which is `Duration::ZERO`
at the tag, same as the PostgreSQL counterparts held back in v26.47.0.
If the flag is served on, both halves want announcing together.
- **MCP servers GA**
([#40448](#40448)):
omitted as Borderline — the preview banners are gone, but the GA call
belongs to the release owner. Publish it?
- **Balancer connection metrics**
([#40192](#40192)):
omitted as Borderline — `mz_balancer_connection_status{status="error"}`
changes meaning, so Self-Managed dashboards and alerts on that series
will see a step change. Worth a note?
- **Console query comments**
([#40075](#40075)):
omitted as Borderline — every Console statement's `sql_hash` changes
once when this deploys. Worth calling out?

Every flag above was re-read in source at the `v26.50.0` ship tag; not
one default moved between `rc.1` and ship, so all six holds still read
correctly.

Snapshot drafts (with PR links) in
[mz-skills](https://github.com/MaterializeInc/mz-skills):

- rc.1:
[release_notes.md](https://github.com/MaterializeInc/mz-skills/blob/main/data/mz-release-notes/v26.50.0/rc.1/release_notes.md),
[omitted.md](https://github.com/MaterializeInc/mz-skills/blob/main/data/mz-release-notes/v26.50.0/rc.1/omitted.md)
- final:
[release_notes.md](https://github.com/MaterializeInc/mz-skills/blob/main/data/mz-release-notes/v26.50.0/final/release_notes.md),
[omitted.md](https://github.com/MaterializeInc/mz-skills/blob/main/data/mz-release-notes/v26.50.0/final/omitted.md)

**Borderline PRs omitted:** rc.1: 7 —
[omitted.md#borderline](https://github.com/MaterializeInc/mz-skills/blob/main/data/mz-release-notes/v26.50.0/rc.1/omitted.md#borderline),
final: 0 —
[omitted.md#borderline](https://github.com/MaterializeInc/mz-skills/blob/main/data/mz-release-notes/v26.50.0/final/omitted.md#borderline)
(no PRs in scope; `rc.1`'s six holds re-examined at the ship tag and
unchanged)

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

---------

Co-authored-by: Claude <[email protected]>
Co-authored-by: Pranshu Maheshwari <[email protected]>
Co-authored-by: Valerie Chiang <[email protected]>