feat: use R from synchronized rig projects

[eitsupi]

• Add rig-project to --r-provider, ARF_R_PROVIDER, and experimental.r_source_overrides so external tools can select a synchronized project without user-specific arf configuration. Explicit requests fail without fallback; configured providers retain ordered, best-effort resolution.
• Pin the verified project wrapper, R_HOME, and shared library, and preserve the selected provider and target across Unix loader re-execution so startup cannot silently switch installations. Delegate project-library activation to normal R startup and rig's .Renviron machinery.
• Report project identity independently from selection provenance in the shared resolve/headless JSON, with stable failure reason codes for external tools. Keep rig installation names separate from exact R versions.
• Enable :switch and completion for rig-project sessions so switching becomes an explicit installed-version selection, while subsequent restarts retain that selection and Pixi switching remains disabled.
• Run real rig-project startup, compiled-package loading, and CLI/environment resolution on Linux, macOS, and Windows using pinned mise tools. Keep the detailed three-version switching and DYLD precedence checks on macOS, and reuse portable fixtures for strict failures, override selection, and fatal library-loading failures. Update schema, help, completions, and documentation alongside the public interface.
• Document the rig 0.11.0 limitation for early experimental availability: crossing R minor versions can leave an incompatible project library active despite a warning (Project library remains active after an R minor-version mismatch r-lib/rig#450). Keep switching available and advise avoiding cross-major/minor switches until using a rig release containing Do not activate project for R version mismatch r-lib/rig#451 and running rig proj sync. Track the fixed release's CI pin update, project resynchronization, and restoration of the held assertions as follow-up work.