Bootstrap V4 differential conformance campaign

[OttoApocalypse69]

What changed

• add a strict V3/V4 differential manifest and runner with four initial semantic cases
• build a TY-only V4 campaign probe that embeds source opaquely, validates byte length/digest, and fails closed on native/runtime claims
• rebuild current V3 through the canonical seed-to-stage2-to-CLI sequence rather than trusting a stale binary
• enforce 11 phased campaign categories, including explicit empty phases, independently from compatibility relationships
• reject duplicate manifest/probe JSON keys and bound complete process trees on Windows and POSIX
• add a dedicated Ubuntu CI job for the campaign and nested POSIX cleanup self-test

Boundary and claims

This bootstrap compares frontend accept/reject/category behavior only. It reports v4-native=0 and never fabricates native equivalence. The V4 probe stops at TY and does not reach MIR, codegen, or target execution.

Validation

• independent exact-head review of 3180fb5f36618f33259bb859d1665c5ea0b98676: APPROVE, no P0-P3
• freshly rebuilt current V3 + V4 campaign: 4/4 pass, zero mismatches
• LLVM-MinGW generated-probe self-test: 3 compiled probes
• manifest self-test: 4 cases, all 11 ordered categories, phase mapping, duplicate-key rejection
• Windows Job Object nested timeout cleanup: pass
• independent WSL/Linux nested memory cleanup and outer-timeout probe: pass
• Python compilation, AST/static boundary checks, and git diff --check: pass

Stack and pending gates

This draft is based on fix/v4-post-merge-contract-hardening / PR #91 and is independent of the MIR split. Current-head Ubuntu CI remains required for the committed POSIX aggregate memory/termination path; macOS process-accounting fallback remains a platform evidence item. No existing PR readiness was changed.

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