Release v0.9.0

[hdamker]

Problem description

Cut the numbered tooling 0.9.0 (minor) release. Consolidates the changes merged to main since v0.8.1, tracked in milestone v0.9.0 (24 items, 0 open). Adds externalDocs validation against Design Guide §5.4 (new rules P-038/P-039) and a bundler component-renaming-collision check (new rule P-040); see the milestone and the v0.9.0 CHANGELOG.md section for the full list of changes.

Expected action

• Confirm Validation Regression canary green at the release HEAD (b1d6fc2, run 31169997789, 12/12).
• Confirm Release Automation Regression canary green (no RA-scope commits since the last green run).
• Open the release metadata PR (VERSION.yaml, CHANGELOG.md v0.9.0 section, README tag reference): Prepare tooling 0.9.0 release #420.
• Merge the release metadata PR.
• Publish the v0.9.0 GitHub Release (Latest) on the merge commit.
• Confirm the post-merge Validation Regression canary is green.
• Move the v1-rc floating tag from 6b29358 to the v0.9.0 release commit.

Additional context

v0.8.1 was published on 2026-07-24 and currently anchors v1-rc. No functional changes touch release_automation/, shared-actions/, tooling_lib/, or .github/workflows/ beyond a mechanical NO_COLOR env addition in validation.yml (#414); canary coverage is accepted as sufficient gate evidence for that change, matching the mechanical-change precedent from v0.7.0/v0.8.0.