Add cross-language readiness checks and offline fixture probes

[haddanki-personal]

Summary

Adds a shared readiness stage before expensive instrumentation and analysis, covering .NET, Java, Node/TypeScript, Go, Rust, and Python.

• Add realdiff doctor <repo> [--json] [--out <file>] [--probe] through the native and managed entry points.
• Check actual base and PR revisions independently before builds or baseline-cache reuse; support --readiness-probe for analysis and warm.
• Inspect installed toolchain selection and tracer assets without building project code or provisioning toolchains.
• Ship opt-in offline fixtures using the production instrumenters. Require nonempty trace/manifest pairs, expected argument/return evidence, source attribution, per-process root correlation, and reconciled counters.
• Include verified compiler/runtime/runner context in versioned baseline-cache keys.
• Preserve readiness.json through CLI failures, MCP, and CI/action integrations.

Repository -> installed tools + assets + compatibility evidence
                               |
                   optional bundled fixture
                               |
                    ready / blocked + reason
                               |
                     existing analysis pipeline

Intentional behavior changes and limits

• Required failed or unknown checks block analysis. Opaque custom build/test commands and unresolved Java wrappers remain blocked.
• Old baseline-cache entries become misses.
• Probes are explicitly authorized, use local dependency caches, and do not silently download missing prerequisites. Temporary directories are not a security sandbox.
• Passing readiness is not a clean-PR verdict or proof of compatibility with every source shape. This exposes compiler compatibility; it does not remove every compiler dependency.

Validation

• Readiness proof: 113 assertions, including negative trace cases and Windows script quoting.
• Native launcher: 18 unit tests; native doctor exercised for all six languages.
• Installed package: all six readiness probes passed; Java, Node, Go, Rust, and Python reference analyses completed successfully.
• Self-contained .NET fixture probe passed.
• Same Go rewriter exercised with Go 1.27.1 and 1.27.0; Rust rewriter exercised with Rust 1.98.1 and 1.97.0.
• Rust cold/warm/cache-hit behavior and toolchain-change invalidation exercised.
• Managed detection, configured-command refusal, CLI/MCP builds, and diff hygiene checked.

Local execution was on Windows using SDK 10.0.401 because the repository-pinned SDK 8.0.424 was not installed. Windows/Linux CI coverage is configured; Linux was not executed locally. The existing native detect Gradle-marker gap remains outside this change; managed detection and doctor routing are separate from that limitation.