refactor: share mechanical real-binary CLI test fixtures - #399

What this changes

Refs #332, refs #145. Share temporary store ownership, cleanup, real CLI invocation and bounded JSON decoding across assembly, bolt, weld and level integration suites. Keep domain setup, independently expected results and assertions in each suite. Reserve directories atomically using PID and a counter; skip existing names with a bounded retry and never reuse their contents.

Independent of #396, #397 and the publication slice. Open #335 fixes the separate host recovery fixtures; this PR does not duplicate or modify that file. Open #243 modifies other CLI test suites, which remain untouched here.

How you verified it

• cargo fmt --all -- --check
• git diff --check
• Sequential combined verification of refactor: share CLI result reporting outside plate commands #396-refactor: share mechanical real-binary CLI test fixtures #399: workspace clippy with -D warnings and 107 tests across 17 binary/integration suites passed. The suites include assembly, bolt, weld, level, numbering approval, publication guard/evidence, command evidence, recovery, rebase, workspace, BOM, changeset, coordinator, drawing codec and picking.
• Full platform CI passed at 8850eee025bffd26ea9aa9619122549cb4b8d4e4: run 40181401596.

The earlier overlapping local workspace/test pipelines were interrupted and are not claimed as isolated full-suite passes. The combined verification branch is local only; each PR retains its own changes. No manual native UI acceptance is claimed.

Never-crash

• Only test infrastructure changes; production input handling unchanged.
• Cleanup is scoped to the newly reserved fixture directory.

Provenance

• Original Rust refactoring; existing contributor-original fixtures retained.
• No new assets, dependencies or commercial-product/GPL/AGPL material.

Anything a reviewer should look at first

Assertions and domain-specific fixture methods are intentionally kept separate. Existing temporary directories are never adopted or removed.