Complete the batch compiler, MSBuild, and transactional artifact contract

Parent: #8
Blocked by: #25, #26, #38, and #39

Outcome

Complete the observable batch-compiler and Compiler Target Invocation contract around the generic compiler core.

Acceptance criteria

• Response files and the declared command-line option matrix normalize into the compiler request without silently ignored semantic options.
• The Fsc task and CoreCompile integration preserve ordered sources, references, defines, target-framework-selected inputs, command-line capture, exit propagation, and compiler-owned artifacts.
• Standalone and persistent-service adapters produce equivalent diagnostics and artifacts for the same request.
• Successful artifact sets publish transactionally; failed or unsupported requests preserve the last successful set and clean temporary state according to the Compatibility Oracle contract.
• The opt-in NuGet integration selects the packaged NativeAOT host without source edits or production fallback.
• Generic multi-project and multi-target fixtures validate behavior independently of the first real-world corpus.
• Packaging and platform expansion beyond the compiler workstream remain tracked by #1 decisions.

Decision inputs

Issues #2, #4, and #5; ADRs 0003, 0004, 0009, 0010, and 0019.

This issue inherits the corpus-neutral acceptance policy in #8.

Start by reading the generic compiler core, the Fsc task, and the CoreCompile integration, then review issues #25, #26, #38, #39 and ADRs 0003, 0004, 0009, 0010, and 0019. Use the acceptance criteria as the definition of done: normalized requests, equivalent adapters, transactional artifacts, opt-in NuGet integration, and independent multi-project and multi-target fixtures.