Make SpecificationItem immutable with toBuilder()

[kaklakariada]

Goal

Make SpecificationItem fully immutable by replacing its public mutation API with a builder-copy workflow.

Scope

• Remove the public SpecificationItem.addCoveredId(SpecificationItemId) method as a breaking API change.
• Add SpecificationItem.toBuilder(), returning an independent builder pre-populated with every item property, including located IDs/ranges, text fields, status, location, relations, tags, and forwarding state.
• Defensively snapshot all builder-owned collections when building a SpecificationItem, so later builder changes cannot affect an existing item.
• Preserve the linker’s derived covered-ID behavior: when it must add a missing covered ID, LinkedSpecificationItem replaces its wrapped item with item.toBuilder().addCoveredId(...).build() instead of mutating the original.
• Update dsn~specification-item to revision 4 and update corresponding implementation and unit-test trace markers.

Acceptance Criteria

• A toBuilder().build() copy retains all source properties.
• Changing a copied builder produces a new item and leaves the source unchanged.
• Mutating a builder after build() does not change the built item’s collections.
• Linked items continue to expose inferred covered IDs after linking, while the source SpecificationItem remains unchanged.
• ./oft-self-trace.sh and mvn -T 1C verify pass.