feat: consume pinned cxcc release

[Tim-1e]

Summary

• switch dotfiles from bundled cx/cc implementations to pinned cxcc v0.1.0
• verify immutable installer and platform artifact digests before installation
• load PowerShell and Zsh through stable cxcc loaders while preserving existing user state
• replace duplicate implementation tests with consumer, full-install, and public-hygiene coverage

Impact

Existing cx, cc, and mcp commands keep their current state and configuration paths. Set INSTALL_CXCC=0 to skip one apply; a later apply installs normally.

Validation

• removed the verified legacy Windows helper, uninstalled cxcc, then installed from this dotfiles branch twice
• confirmed cx, cc, and mcp in a fresh PowerShell profile
• confirmed six protected user-state hashes and repeated-apply hashes remained unchanged
• passed PowerShell/Shell consumer, profile, Windows full, font, Termux, syntax, hygiene, and Gitleaks checks locally
• GitHub Actions clean-runner validation is pending on this PR