Support notes-product-version.json indices

[lcawl]

Summary

This PR adds product-scoped notes indexes that are additive.
To ensure we don't break existing changelog users, the legacy notes-{version}.json keys stay, and {changelog} still does not read those indexes.

Background

Relates to #4090, #4103

Since the related issues touch the same areas of the code and have a significant, overlapping impact, this PR is part of a multi-PR phased approach.

Details

Indexes. The scrubber now writes changelog/{org}/{repo}/notes-{product}-{version}.json (with product and version on the body) and still dual-writes notes-{version}.json as a version union without those fields. Stale product-scoped keys can be deleted; a version-only key is kept while any note still declares that version.

Consumers. changelog bundle GETs the product-scoped URL first and falls back to the version-only URL only on HTTP 404. An empty product-scoped index does not fall back. MergeNotesAsync walks each (product, version) from --output-products and still dedups by checksum.

Docs (same change set). docs/development/changelog-bundle-registry.md, docs/cli/changelog/cmd-note.md, and docs/cli/changelog/cmd-backfill.md.

The dual-write path is in the changelog-scrubber Lambda. It will not show up on the live CDN until that Lambda is deployed.

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

2. If you answered "Yes" to the previous question, please specify the tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).

Tool(s) and model(s) used: Cursor Grok 4.6

[github-actions]

Docs preview (local build)

Handbook preview: https://docs-v3-preview.elastic.dev/elastic/docs-builder/pull/4120/

Sample changed pages:

• docs/cli/changelog/cmd-backfill.md
• docs/cli/changelog/cmd-note.md
• docs/development/changelog-bundle-registry.md

[github-actions]

Requesting changes: the current product/version notes flow can surface wrong-product notes in both CDN fallback and amend reconciliation paths.

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