AI visibility: canonical audit of Core engine-concept pages

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## Goal

Verify `layouts/partials/header/canonical.html` produces the correct canonical URL on Core engine-concept pages after recent shared-source changes. Engine-level concepts shared between Core and Enterprise should canonical to Enterprise; Core-specific landing/install/quickstart/upgrade pages should self-canonical.

Parent: #7230 (Phase 0)

## Context

The canonical partial has a selective-opt-in mechanism (sentinel `canonical: self`, plus a priority loop over product paths for shared-source pages). Behavior depends on which `source:` paths are configured in the priority list and on the per-page `canonical:` value. This is a verification task — find drift between intent and rendered HTML.

## Page-class taxonomy

- **Engine-level concepts** (storage, indexing, partitioning, compaction internals): canonical → Enterprise
- **Core-specific** (Core landing, install, quickstart, upgrade-to-Enterprise): self-canonical
- **Enterprise-only** (HA, replicas, cluster admin): self-canonical (already correct by default)

## Acceptance criteria

- Audit every \`content/influxdb3/core/**/*.md\` page (regular pages and \`_index.md\` stubs)
- Document each page's class and expected canonical
- Cross-check against the rendered \`\` in \`public/influxdb3/core/**/index.html\`
- File a follow-up PR (or PRs) for any page where the rendered canonical doesn't match the page class — either by setting \`canonical: self\` in the page's frontmatter or by adjusting the priority list in \`layouts/partials/header/canonical.html\`
- Report findings as a table in this issue's first comment (page → expected → actual → action)

Audit every content/influxdb3/core/**/*.md page, including _index.md files, and inspect the matching public/influxdb3/core/**/index.html output. Read layouts/partials/header/canonical.html first, classify each page as engine-level, Core-specific, or Enterprise-only, and record page, expected canonical, actual canonical, and action in the issue’s first-comment table. Done means all pages are checked and any mismatch has a follow-up PR or is documented as correct.

Written by the indexing model from the issue text.