Preserve selected nested test environments

[Krastanov-agent]

Closes #97.

Summary

• Treat a canonical strict-descendant projectUri as the complete test environment.
• Copy it to scratch, bind the owning package checkout, and preserve selected manifest paths, versions, and preferences.
• Keep the existing TestEnv.activate path for equal, enclosing, sibling, external, missing, and cross-drive paths.
• Add a network-free controller fixture and document the activation split.

This intentionally changes the strict-descendant case that #98 currently documents as legacy behavior. Other projectUri relationships retain that behavior.

Validation

• Focused scratch and controller tests: 80/80
• Full Pkg.test(): 1161/1161
• Nested fixture on Julia 1.0 through 1.12: 9/9 on every minor
• Controller documentation build
• QuantumClifford without the load-path workaround: base 59/59, static analysis and doctests 11/11, ECC 18/18

GPU suites were not run because matching hardware was unavailable.

[Krastanov]

This unreviewed unvetted slop fest is my submission for #97

I wanted to see whether the problem is simple enough to be well one-shot by codex (sol 5.6 ultra). It happens to work well with the multi-project.toml test environment I need for QuantumClifford.jl. But I am not familiar with this codebase or with the "spirit" of how its maintainers would like it to work, so I am NOT yet standing by this code.