v26.15.0 release notes

[bosconi]

For included items with PR links see: https://github.com/MaterializeInc/docs-automation/blob/main/release-notes/output/release_notes_v26.15.0.md

For omitted items with PR links see: https://github.com/MaterializeInc/docs-automation/blob/main/release-notes/output/release_notes_v26.15.0_omitted.md

Borderline PRs omitted: 6 — review borderline calls

[github-actions]

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[cursor]

PR Summary

Low Risk
Documentation-only update adding the v26.15.0 release entry and Self-Managed operator compatibility row; no product code changes.

Overview
Adds the v26.15.0 release notes entry to releases/_index.md, covering new Self-Managed cpu_request sizing support, Console label rename, and several bug fixes (e.g., AS OF validation, Kafka sink auth edge case, COPY FROM STDIN panic, and Console UI issues).

Updates self_managed_operator_compatibility.yml to include v26.15.0 operator/orchestratord/environmentd versions and release date with a link to the new release notes section.

^{Written by Cursor Bugbot for commit 351ba60. This will update automatically on new commits. Configure here.}