Add cross-language differential verification

[Jason-Doyle]

Summary

• add a required, dependency-free cross-language harness that compares complete TypeScript, Go, Python, and .NET CLI results over one byte-identical corpus
• cover every committed vector and example plus malformed transport, numeric/Unicode/planner edges, digest-equivalent encodings, and 500 deterministic randomized inputs per PR
• fail closed on runtime/process errors, enforce conformance corpus hashes and expected outputs, type-check the harness, bound builds/invocations, and provide reproducible seed/filter diagnostics
• reject malformed UTF-8 in the TypeScript CLI instead of silently replacing bytes, and harden Go finite-underflow handling without accepting overflow

Quality gate

• default seed: 583/583 cases agree across all four implementations
• audit seed cross-language-audit-2026-09: 2,583/2,583 cases agree
• 69 Node tests, 54 Python tests, and 548 .NET tests pass
• Go unit/vet/race checks pass, including focused number parsing tests
• root release/package checks and all language package smokes pass
• actionlint, strict TypeScript/JSDoc checking, Ruff, mypy, .NET analyzers, and vendored-source verification pass