feat(collections): standardize maturity field across all 14 collection YAML manifests

[MauroDruwel]

Summary

Closes #1319

Audited all 14 collection YAML manifests against docs/docusaurus/src/data/collectionCards.ts and resolved every gap, conflict, and placement inconsistency identified in the issue.

Changes

1. Collection YAML manifests — maturity field added/corrected

maturity: is now placed consistently after description: across all 14 manifests.

Collection	Before	After	Source
ado	(missing)	stable	Docusaurus
coding-standards	(missing, plugin-level: experimental)	stable	Docusaurus
data-science	(missing, plugin-level: experimental)	stable	Docusaurus
design-thinking	preview (wrong position — after tags)	preview (after description)	Already set
experimental	experimental ✓	unchanged	Already set
github	(missing)	stable	Docusaurus
gitlab	(missing)	experimental	Docusaurus
hve-core	(missing)	stable	Docusaurus
hve-core-all	(missing)	stable	Meta-collection wrapping stable content
installer	(missing)	stable	Meta-collection wrapping stable content
jira	(missing)	experimental	Docusaurus
project-planning	(missing, plugin-level: experimental)	stable	Docusaurus
rai-planning	experimental ✓	unchanged	Already set
security	(missing, plugin-level: experimental)	experimental	Docusaurus

2. Validate-Collections.ps1 — maturity is now required

Changed maturity: from an optional field (silently passes if absent) to a required field. Before: the validator only checked values when the field was present. After: missing maturity: field triggers a validation error with message missing required field 'maturity'.

The new check follows the same pattern as other field validations (id, name, description) and uses the existing $allowedMaturities set: stable, preview, experimental, deprecated.

3. Validate-Collections.Tests.ps1 — tests updated for new behavior

• Changed "Passes validation for collection with omitted maturity" → "Fails validation for collection with omitted maturity" to match enforced behavior.
• Added maturity = 'stable' to all other test manifests that were constructed without a maturity field (these tests were verifying unrelated behavior — path validation, folder naming, marker parsing — and needed the field to satisfy the new requirement).

Conflict Resolution: Collection Maturity vs. Plugin-Item Maturity

Three collections (coding-standards, data-science, project-planning) show experimental at the plugin-item level but Stable in Docusaurus. This PR sets collection-level maturity to stable matching Docusaurus, for the following reason:

Collection maturity represents the collection's distribution readiness (is it packaged, tested, and safe to install?), not the stability of every individual artifact it contains. These three collections are production-distributed, well-maintained packages. Individual items that are experimental carry their own item-level maturity: field which downstream consumers (Prepare-Extension.ps1 maturity filtering, #1317 badge rendering) already read separately.

Meta-Collection Decision

hve-core-all and installer are not present in Docusaurus. Both are packaging meta-collections wrapping stable production content, so stable is the correct value.

Testing

Command	Result
npm run lint:collections-metadata	✅ 14 collections validated, 0 errors
npm run test:ps -- -TestPath scripts/tests/collections/	✅ 105 tests passed, 0 failed
npm run plugin:generate	✅ 14 plugin(s) generated

Manual testing was not performed.