refactor(tauri): split runtime helpers from lib.rs

[SuperJJ007]

Summary

• Split Rust runtime helper code out of desktop/src-tauri/src/lib.rs into focused runtime/* modules.
• Keep this scoped to B7 public-safe engineering cleanup.
• Add focused test runner entrypoints for Rust, contract, lifecycle, and live validation layers.
• Rebased the draft PR branch onto current main so the PR diff is a single B7 commit.

Verification

• git status --short --branch
• git diff --check
• git merge-base --is-ancestor 00b28b0 HEAD; echo $? -> 1
• git merge-base --is-ancestor 160d815 HEAD; echo $? -> 1
• bash test/run-rust.sh -> Rust 127 passed
• bash test/run-contract.sh -> offline 126 passed + Rust 127 passed
• bash test/run-lifecycle.sh -> loopback 24 passed + scripts/oauth/ops pass

Boundaries

• This is B7 Rust/Tauri lib.rs engineering cleanup.
• This is not S1a Python compat implementation.
• This PR is based on current main; it does not introduce OpenAI Responses support, but preserves the support already present on main.
• This does not touch private docs or version numbers.
• This does not claim full release readiness.
• This does not claim real-machine live validation.
• This does not claim desktop packaging/install-path validation.

Notes

test/run-lifecycle.sh still emits Python socket ResourceWarning lines in the known non-blocking class, but exits 0.