fix(verso): route project output to isolated fragment

[imathwy]

Problem

Project finalizers configured VERSO_OUTPUT_DIR but the SDK only verified that path after running lake exe reasbook-site. Verso therefore wrote to its default _site directory, and the isolated fragment finalizer failed after otherwise successful generation.

Fix

• pass the normalized output path as the generated Verso executable argv: --output PATH
• reject ambiguous explicit output flags and non-executable targets
• exercise the real SDK from the repository fragment test with a fake executable that only honors argv
• document the output-isolation contract

Verification

• Verso SDK: 34 tests
• all capability SDK tests
• repository tests: 50
• SiFlow adapter tests: 175
• ruff, Python compilation, and git diff checks

No Lean build was run locally. The remote release will reuse the immutable branch Lean caches.