[feature] Add skipped inventory aggregates for issue 267

[paul-fresquet]

Summary:

• Add skipped-entry aggregates to inventory models to support reliable cross-machine reporting.
• Keep runtime skipped tracking while introducing persisted per-part skipped counts.

Key changes:

• Added persisted skip aggregates in InventoryPart (SkippedCountsByReason, SkippedCount, GetSkippedCountByReason, RecordSkippedEntry).
• Extended InventoryProcessData with runtime aggregate counters (SkippedCount + per-reason counts) and reset handling.
• Updated InventoryBuilder.RecordSkippedEntry to write through a single path: InventoryPart aggregate + runtime skipped entry/counters.
• Added/updated unit tests for InventoryPart aggregates, process data counters, builder propagation, and loader persistence.

Notes/risks:

• Existing inventory files without skipped aggregates remain compatible (default zero values).
• No UI changes are included in this PR (preparation for issue [feature] Display skipped entries counter in inventory UI #268).

Closes #267