Mark shared Wave cases as compile-portable or runtime-portable

## Problem

PR #457 makes `shared/` compile on every enabled target, but QEMU and WebAssembly runtime selection cannot safely execute every shared case. Some otherwise portable sources require process I/O or custom WebAssembly host imports. The current runner therefore executes only `shared/` and `/` for those targets.

## Scope

- Define an explicit, backend-neutral contract for cases that are safe to execute on a target runtime.
- Distinguish compile portability from runtime portability and required host capabilities.
- Make `runtime-sources` include eligible root `shared/` cases.
- Keep missing browser host imports aligned with #453 and target capability diagnostics aligned with #451.
- Add unit tests for native, QEMU, bare WebAssembly, and WASI selection.

## Completion criteria

- Self-contained shared cases run under every compatible enabled target.
- Cases requiring unavailable host services remain compiled but are excluded or reported as capability skips.
- Selection is manifest/metadata driven, not based on filenames or source-text heuristics.

Start by tracing the runtime-sources selection and the existing shared/ and / paths. Review how target capabilities, browser host imports, and capability diagnostics relate to #453 and #451, then add unit tests for native, QEMU, bare WebAssembly, and WASI selection. Done means metadata-driven selection includes compatible root shared/ cases while skipping or reporting unavailable host services.

Written by the indexing model from the issue text.