Group diff output by process group

[sfc-gh-pvillard]

Closes #75.

Summary

• Group headers per process group: each process group involved in changes gets its own **\Root > SubGroup`** — N changes` header, making it immediately clear which group a change belongs to. Groups are sorted alphabetically; the root group comes first, nested groups in path order.
• (Parameter Contexts) section: parameter-context-level diffs (value changes, parameter adds/removes inside a context) are collected in a dedicated section printed last, separate from structural process group changes.
• Snapshot A process groups registered: groups from the before-snapshot are now also registered so that components removed from groups that no longer exist in the after-snapshot still display a human-readable path instead of a raw UUID.
• No change to per-diff message wording: every existing diff description is unchanged; only the grouping and headers are new.
• Controller service added message simplified: since the group is already shown by the section header, the redundant has been added in Process Group X suffix was removed from the controller service added message.

Example output

#### Flow Changes

**`MyFlow`** — 5 changes
- A Processor of type `GenerateFlowFile` named `Gen` has been added ...
- In Processor of type `UpdateAttribute` named `Update`, the value of property ...

**`MyFlow > Ingestion Group`** — 2 changes
- In Processor of type `InvokeHTTP` named `Call API`, the Run Schedule changed from `1 min` to `* * * * * ?`
- A Processor of type `UpdateAttribute` named `Enrich` has been removed

**`(Parameter Contexts)`** — 1 change
- In the Parameter Context `MyCtx`, the value of the parameter `api.url` has changed from `https://old` to `https://new`

Test plan

• All 26 existing tests continue to pass (mvn test)
• testNestedGroupsHaveSeparateSections — verifies the raw diff set contains expected diff types when using the new nested-PG fixtures
• testGroupedOutputContainsGroupHeaders — verifies group headers appear in the correct order and changes are placed under the right group
• testSingleGroupOutputHasGroupHeader — verifies single-group flows still produce a group header and no spurious (Parameter Contexts) section