Fix changelog note indexes so they're product-scoped

[lcawl]

What happened?

Notes indexes are keyed by version only: changelog/{org}/{repo}/notes-{version}.json. Neither the object name nor the JSON body records which product the notes belong to.

For example, a cloud-enterprise 4.2.0 note uploaded with docs-builder changelog upload --artifact-type changelog produced:

{
  "schema_version": 1,
  "notes": [
    {
      "path": "master/note-the-list-of-supported-operating-systems-has-been-updated.yml",
      "bundle_seq": 2
    }
  ]
}

with filename notes-4.2.0.json. Expected: the index is scoped to the product (and version) declared on the note, e.g. notes-cloud-enterprise-4.2.0.json, with product / version in the body so a downloaded file is self-describing.

This is more than a naming issue. NotesIndexReconciler reads products[].product and products[].versions, then drops the product and unions version strings. NoteAmendReconciler then matches all notes for that version against every product that has a bundle at the same version. changelog bundle CDN note fetch does the same (notes-{version}.json; output products are collapsed to distinct versions).

The cloud repo authors several products (cloud-enterprise, cloud-hosted, cloud-serverless, cloud-kubernetes, …) that can share version or date slugs. An ECE-only 4.2.0 note can be treated as a late note for another product’s 4.2.0 bundle (wrong .amend-notes sidecar and bundle_seq).

How can we reproduce it?

1. Author a note that applies to one product at a version another product in the same repo might also use:

   products:
   - product: cloud-enterprise
     versions:
     - 4.2.0

2. Upload with docs-builder changelog upload --artifact-type changelog --target s3 ….

3. Inspect the public notes index: changelog/{org}/{repo}/notes-4.2.0.json — no product in the key or payload.

4. (Cross-product) Publish a bundle for a different product at 4.2.0 in the same repo. The ECE note is eligible for that product’s amend reconcile / CDN note merge.

A note that lists several products[] / versions should appear in one index per (product, version) pair; the YAML in the changelog pool stays a single file (same fan-out as multi-product bundles).

Version or commit

Current main (notes index layout from ChangelogKeys.NotesIndexKey / NotesIndexReconciler; not a one-off CLI regression).

Additional notes

I believe that this issue will implement changes that overlap with #4090 so the fixes will need to be coordinated

[lcawl]

Per AI analysis, a possible solution is as follows:

Index each changelog note by each (product, version) pair it relates to:

Note YAML	Indexes written
cloud-enterprise × [4.2.0]	notes-cloud-enterprise-4.2.0.json
same note, also cloud-enterprise × [4.1.0]	also notes-cloud-enterprise-4.1.0.json
same note, also cloud-kubernetes × [4.2.0]	also notes-cloud-kubernetes-4.2.0.json

This is the same fan-out already documented for bundles in docs/cli/changelog/cmd-upload.md: one artifact, multiple product keys.
Multi-version fan-out already exists (ExtractVersionsAsync + “a note may declare multiple versions, so one note can appear in several indexes”).
Product is the missing dimension.

A note with no usable products[].versions (and no legacy target) still produces no index, as today.

Key layout (keep a direct child of {org}/{repo}/)

Use changelog/{org}/{repo}/notes-{product}-{version}.json.

• Stays a single path segment after {org}/{repo}/, so ChangelogKeys.IsNotesIndex (no extra slashes) and ChangelogScope.TryFromKey keep working.
• Construct, do not parse the slug. Product IDs contain hyphens (cloud-enterprise); versions can too (2026-05-15). Stale deletion should compare full remaining slugs (cloud-enterprise-4.2.0) against the set of keys this pass intends to write, not split on -.
• Sanitize product and version the same way existing version slugs are validated (IsValidSegment, skip /).

Do not nest under notes/{product}/… unless we later want a layout change; that would break the current notes-index detector.

Self-describing JSON (additive fields)

Keep schema_version: 1. Add top-level product and version on NotesIndex, matching bundle registry’s product field. Body stays paths + bundle_seq:

{
  "schema_version": 1,
  "product": "cloud-enterprise",
  "version": "4.2.0",
  "notes": [
    { "path": "master/note-the-list-of-supported-operating-systems-has-been-updated.yml", "bundle_seq": 2 }
  ]
}

bundle_seq becomes per product×version, which is the correct meaning: shipped in ECE’s 4.2.0 bundle vs still late for Kubernetes 4.2.0.

Code changes

• ChangelogKeys.NotesIndexKey(org, repo, product, version) — composite slug. IsNotesIndex still accepts any non-empty slug after notes-.
• NotesIndexReconciler — group as (product, version) → entries; ExtractVersionsAsync becomes extract (product, version) pairs (still prefer versions, fall back to legacy target). DeleteStaleIndexes drops old notes-{version}.json keys automatically because 4.2.0 will not be in the new key set.
• NoteAmendReconciler — notesByProductVersion; ProcessProductAsync only sees notes for that product. Rewrite indexes with product-scoped keys.
• CdnChangelogEntryFetcher.FetchNotesAsync — take product; GET notes-{product}-{version}.json.
• MergeNotesAsync / ResolveNoteTargets — iterate distinct (OutputProducts.Product, Target) pairs, not versions alone.
• ScrubberProcessor comments only (still reject client-uploaded notes-*.json).
• Local backfill NotesRegistry already lives under {output}/{product}/changelog/; leave it unless we want filename alignment for humans. It is not the CDN contract.

No CLI flags. No feature flag: this is a correctness fix, and flag-off would keep the cross-product amend bug. One Lambda + docs-builder release; the next note reconcile (or a re-upload with --skip-etag-check) rebuilds public indexes and deletes stale version-only files.

Tests (default: dotnet test tests/Elastic.Changelog.Tests/)

Add / extend:

• NotesIndexReconcilerTests: two products same version → two keys; one note two versions → two keys; one note two products × two versions → four keys; stale notes-4.2.0.json deleted.
• NoteAmendReconcilerTests: ECE note + ECE and Kubernetes 4.2.0 bundles → amend only under bundle/cloud-enterprise/; bundle_seq independent per index.
• CdnChangelogEntryFetcherTests + BundleCdnSourcingTests: fetch URL includes product; bundling two output products at the same version hits two indexes.
• ChangelogKeysTests if key helper coverage exists.

Also tests/Elastic.Documentation.Configuration.Tests/ for the fetcher/keys change.

Docs

• docs/development/changelog-bundle-registry.md notes-index format and key.
• docs/cli/changelog/cmd-note.md “after creation”.
• docs/cli/changelog/cmd-backfill.md only if we mention the CDN key (backfill local path is already product-scoped).
• Grep elastic/docs-actions for hardcoded notes-{version}; matching PR only if those wrappers construct the URL (skill: CI-facing CLI). Unlikely — indexes are Lambda-owned.

[lcawl]

Per further AI analysis, this fix is combined with the fixes for #4090 and spans multiple PRs to minimize risk.

Phase	Ships	Must not
1 4104	BundleLoader merges .amend-notes.yaml	—
2 4120 — additive indexes	Dual-write notes-{product}-{version}.json + keep notes-{version}.json. New FetchNotesAsync (product URL, fallback to version-only). MergeNotesAsync iterates (product, version). Docs.	Delete legacy keys. Change NoteAmend’s product walk. Rebuild registry.json.
3 4122 scoped amend + rebuild the product registry	NoteAmendReconciler only sees that product’s notes; returns touched products; ScrubberProcessor re-reconciles those bundle registries and the shallow map.	Delete legacy notes-{version}.json. Sweep other products’ .amend-notes.yaml.
4 4124 cutover	Stop dual-write; then delete leftover version-only objects.	Run before docs-builder that GETs the product URL is what CI/changelog bundle use.