docs: fetch read coalescing (DOC-2386)

[Feediver1]

Resolves https://redpandadata.atlassian.net/browse/DOC-2386

Adds documentation for fetch read coalescing, the opt-in per-shard fetch read coalescer new in 26.2 that removes redundant read CPU and fetch-response memory under high consumer fan-out.

Changes

• New page modules/manage/pages/cluster-maintenance/fetch-read-coalescing.adoc: contextualized intro (the N×→1× redundancy story and an explicit use-when), how it works (coalescing key, the ready-hit/in-flight-hit/miss outcomes, weak retention, per-shard scope), enable procedure (kafka_fetch_read_coalescing_enabled, live toggle, ~0.5 MB/shard cache), an Evaluate coalescing effectiveness section with the four vectorized_kafka_fetch_read_coalescer_* counters and the hit-ratio formula for judging workload fit, and limitations.
• Nav: entry after Follower Fetching under Consume Data.
• follower-fetching.adoc: an ifndef::env-cloud[]-gated TIP cross-linking the new page (that page is single-sourced to cloud-docs; the new page is docs-only, so the cloud build never parses the xref).

All content is sourced from the engineering TOI: engineering/core/toi/v26.2/2026-07-21-fetch-read-coalescing.md (Brandon Allard, SME), and verified against the implementation PR redpanda#31010 (merged to dev 2026-07-13, 26.2-only, no backports): property description, coalescing-key definition, obligatory-vs-strict duplicate-read caveat, and weak-retention behavior all match the source.

Single-sourcing for Redpanda Cloud (added after SME input)

Per Brandon Allard: fetch read coalescing can be enabled on BYOC and Dedicated clusters, so the page is now single-sourced:

• // tag::single-source[] wraps the page; companion stub PR: cloud-docs#645 (draft — must merge only after this content reaches docs main at 26.2 promotion, since cloud-docs consumes streaming from main).
• Cloud build gets: an availability line (BYOC/Dedicated), the enable procedure plus a gated pointer to cloud's cluster-configuration page, and the limitations.
• The Evaluate coalescing effectiveness section is ifndef::env-cloud[]-gated: Cloud exposes only public_metrics to customers, and the coalescer counters are internal vectorized_* metrics. (Worth a Brandon confirmation that there's no Cloud-side visibility planned.)
• The cross-link TIP on follower-fetching.adoc stays ifndef::env-cloud[]-gated for now; un-gating it is a follow-up after the cloud stub merges (otherwise the cloud build gets an xref to a page that doesn't exist there yet).

Review notes

• kafka_fetch_read_coalescing_enabled is not yet in the generated cluster-properties partial (arrives with the 26.2 property regen), so the page mentions it in code font without an xref; the link can be added after regen.
• The four coalescer metrics arrive via the 26.2 internal-metrics regen.
• Deliberately no version callout in prose (the page ships in the 26.2 docs version).
• Per the ticket chain: findings from this write-up inform the final disposition of the Cloud Topics tuning-material ticket (DOC‑2129, written with a non-linking hyphen on purpose — this PR implements DOC-2386 only).

🤖 Generated with Claude Code

Preview pages

• Fetch Read Coalescing (new)
• Follower Fetching (updated)
• Licensing overview (updated: Enterprise features table)

Updates during review

• Enterprise feature (leadership decision 2026-07-23): enterprise-license NOTE + license prerequisite on the page (Self-Managed only) and a new row in the Enterprise features table in get-started:licensing/overview.adoc.
• Moved to Manage > Cluster Maintenance (PM feedback): page now lives at modules/manage/pages/cluster-maintenance/fetch-read-coalescing.adoc, next to Manage Throughput; nav, TIP xref, and licensing-table xref updated; cloud-docs#645 stub mirrored.

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[ballard26]

In addition to my other comments it's likely worth making a note somewhere that fetch coalescing should only be enabled for high fanout workloads. Enabling it for workloads with 1:1 or 1:2 fanout will likely result in higher reactor util due to the overhead the de-duplication logic introduces.

[Feediver1]

Added in 66eed08 — and worth being loud about, since the page previously undersold this as "little-to-no benefit": the enable guidance in both the Self-Managed and Cloud variants now says to enable/request coalescing only for high fan-out workloads because at low fan-out (one or two consumers per partition) the de-duplication overhead can increase reactor utilization, and there's a new Limitations bullet making the same point where readers check before enabling.

[ballard26]

LGTM

[JakeSCahill]

Approving, this is in good shape. The single-sourcing wiring all checks out: I verified develop:consume-data/follower-fetching.adoc and manage:monitor-cloud.adoc both exist in cloud-docs, and the cloud-docs#645 stub mirrors the final page path.

I've left some optional inline suggestions. The main one picks up @ballard26's point about implementation detail in the How it works section: keep the section, trim it to behavior. None of these need to block merge, take or leave them.

Two things I couldn't leave inline because the files aren't in this diff:

1. Release notes: get-started/release-notes/redpanda.adoc on beta covers the other 26.2 features but has no entry for this one yet. Something like this, maybe after the User-based throughput quotas section:

   == Fetch read coalescing
   
   Redpanda can now coalesce concurrent fetch reads that request the same data with the same fetch settings, sharing one read result across all requesting consumers. For workloads where many consumers tail the same partitions, this reduces read CPU and fetch-response memory from roughly N times to one. Fetch read coalescing is an Enterprise feature and is disabled by default. See xref:manage:cluster-maintenance/fetch-read-coalescing.adoc[] to get started.

2. Manage Throughput: it's the natural tuning-story neighbour, so a See also line on manage-throughput.adoc pointing here would help discovery. Note that page is single-sourced to cloud-docs, so gate the link with ifndef::env-cloud[] until cloud-docs#645 merges (same reason as your follower-fetching TIP).

Follow-ups for after the 26.2 regens land: link kafka_fetch_read_coalescing_enabled back to this page via property-overrides.json and swap the code-font property mention here for an xref, and same idea for the four coalescer metrics once the internal metrics regen lands.