feat(scripts): add orphan detection, intra-collection duplicates, companion file, and hve-core-all coverage checks to collection validator

[katriendg]

Summary

Add four missing validation checks to Validate-Collections.ps1 that close gaps between what the validator enforces and what collection integrity requires.

Problem

The collection validator checks manifest-inward only: it verifies declared items exist on disk, IDs are unique, paths resolve, kind-suffix conventions hold, and maturity values are valid. It does not detect:

1. Orphaned artifacts — files on disk not listed in any collection (especially hve-core-all)
2. Intra-collection duplicates — the same kind+path listed twice within a single manifest
3. Missing companion .collection.md — a manifest without its paired markdown description file
4. hve-core-all coverage gaps — items exclusive to a themed collection but absent from the canonical superset

Approach

1. Orphaned artifact detection

PluginHelpers.psm1 already exports Get-ArtifactFiles which discovers all artifacts under .github/ (excluding .github/**/hve-core/). Compare that set against the union of all collection manifest items.

Severity levels:

• Error: Artifact on disk but absent from hve-core-all
• Warning: Artifact only in hve-core-all but not in any themed collection

2. Intra-collection duplicate detection

Get-CollectionItemKey already builds kind|path composite keys. Track keys per manifest and flag when the same key appears twice within a single collection.

3. Companion .collection.md validation

For each .collection.yml in collections/, verify a matching .collection.md (same base name) exists. Report missing companions as warnings.

4. hve-core-all coverage enforcement

Verify every item in any themed collection also exists in hve-core-all. The existing canonical entry check only triggers when an item appears in 2+ collections; items exclusive to one themed collection are never checked.

Acceptance Criteria

• Validator discovers all on-disk artifacts using Get-ArtifactFiles and reports artifacts absent from all collections as errors
• Artifacts on disk but absent from hve-core-all specifically receive a clear error message
• Artifacts under .github/**/hve-core/ remain excluded (repo-specific, not distributed)
• Duplicate kind+path entries within a single collection manifest produce an error identifying the collection ID and duplicated item
• Each .collection.yml without a matching .collection.md produces a warning
• Items in themed collections but absent from hve-core-all produce errors
• Pester tests cover all four new validation scenarios
• CI validation (npm run plugin:validate) catches all new error types

[katriendg]

Dependent on #519 and associated PR merged first.