Validate QEMU smoke cases against their target suites

## Current evidence

`tools/case_manifest.py` derives each target's selected suites as `shared`, `shared/`, and `/`. `smoke_case`, however, is currently validated only as a safe path to an existing file under `tests/cases`.

A QEMU target can therefore name a smoke case from another operating system or architecture and still pass manifest validation. The mistake is only exposed later when CI executes the generated matrix.

## Scope

- Require each `smoke_case` to belong to one of the suites selected for that target.
- Keep the existing safe-path and file-existence checks.
- Preserve valid shared, architecture-shared, and target-specific smoke cases.
- Report the target id and invalid smoke-case path in the manifest error.
- Keep runtime portability selection tracked by #459 separate from this manifest-integrity check.

## Completion criteria

- Unit tests accept smoke cases from valid selected suites.
- Unit tests reject smoke cases from another OS or architecture even when the file exists.
- Existing valid case matrices are unchanged.

In tools/case_manifest.py, trace how target suites are derived and how smoke_case paths are currently validated. Find and run the manifest unit tests first, then extend coverage so valid shared, architecture-shared, and target-specific cases remain accepted while mismatched OS or architecture cases are rejected with the target id and path in the error.

Written by the indexing model from the issue text.