fix(cli): gate Linux-only self-update symbols behind cfg(unix)

[1688mengdie]

Summary

Fixes #2566

Gate the eight Linux-only symbols of the CLI self-update module behind #[cfg(unix)]: the imports flate2::read::GzDecoder / std::io::Cursor / tar::Archive (lines 2/8/12), the DEPRECATION_WARNING const, and the find_package_dir / validate_entrypoint_pair / validate_plugin_host_resources / copy_plugin_host_resources helpers.

The module only performs a real installation on Linux (install_archive is behind #[cfg(unix)] with a #[cfg(not(unix))] stub twin), so on Windows builds rustc reported all eight symbols as unused imports / dead code in the CI logs (see issue #2566 for the run evidence with file:line and rule names).

Type and Areas

Type: bug fix (build warnings cleanup)

Areas: Rust CLI

Motivation / Impact

No direct user-facing change. The Linux self-update path is untouched; Windows/non-Unix builds stop emitting 8 unused-import/dead-code warnings that currently pollute every full CI run of main. This also keeps the codebase ready in case a -D warnings gate is ever introduced.

Verification

• cargo check --locked -p bitfun-cli on Windows: exit 0, warnings for the self_update module gone (baseline 19 warnings for the crate down to 11; the remaining ones are other symbols covered by separate PRs).
• After the change, grep -c '#\[cfg(unix)\]' src/apps/cli/src/self_update.rs reports 9 (8 new gates + the pre-existing install_archive gate); cross-platform live paths (spawn_detached_install, update_from_configured_sources) are untouched.
• linux-only compile surface of the gated symbols depends on the CI ubuntu/macos runners: the Rust Build Check / CLI Tests jobs on ubuntu/macos should confirm the Linux install path still compiles with the gates in place (n/a locally — no Linux/macOS machine available; the platform matrix in CI is exactly the place this gets verified).

Reviewer Notes

• Single file, +8 attribute lines, zero deletions, zero behavior change: the diff only adds #[cfg(unix)] above declarations whose consumers are all inside the #[cfg(unix)] install path.
• No #[allow] suppression is introduced.
• AI-assisted change, lightly tested (cargo check on Windows only; cross-platform behavior verified by CI).

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable. (n/a — no user-facing change)