check: report snapshot items affected by missing packs

[m3nu]

Follow-up from #121. Today vykar check reports [chunk index] pack <id> missing from storage, but it does not tell the user which snapshot items reference chunks from that missing pack.

Goal:

• When one or more packs are missing, report the affected snapshot item paths before the user runs repair.
• Reuse the analysis in check --repair --dry-run so the repair plan shows the concrete item-level impact, not only pack ids.
• Keep healthy repositories cheap: avoid retaining per-item chunk references for all items unless missing packs were found.

Likely implementation:

• Build the missing pack set from the pack existence check.
• Cross-reference missing pack ids to chunk ids via the chunk index.
• Walk snapshot items and emit an item-level issue for items whose chunk_ref.id maps to a missing pack.
• Prefer either checking pack existence before the item walk, or doing a second item pass only when missing packs exist.

Acceptance criteria:

• vykar check still emits the pack-level missing-pack error.
• It also lists affected snapshot/item paths for chunks that reference missing packs.
• check --repair --dry-run includes the same item impact information.
• Tests cover one missing pack referenced by one or more snapshot items.