fix(update): apply staged Windows replacement

[BB-fat]

Supersedes #21 — rebased onto current main (redundant port-fix hunks dropped, composed with #77 auto-update). All credit to @NianJiuZst.

Rebase / composition notes

• The 12-file test port TOCTOU fix (bind 0 instead of probe-then-bind) and the Biome formatting commit were dropped during rebase: both already reached main independently (crates/bsk-cli/tests/cancel_forwarding.rs and friends already bind port 0).
• The core commit applied onto main verbatim — no content conflicts with feat(update): auto-upgrade bsk when daemon finds a newer version #77's update rework. Verified the composition in crates/bsk-cli/src/cli/update.rs:
  • feat(update): auto-upgrade bsk when daemon finds a newer version #77's structure kept: self_install_candidate / replace_binary_at_path (daemon auto-update) and auto_update_step with its AutoUpdateOutcome::Staged mapping.
  • fix(update): apply staged Windows replacement #21's mechanism layered in: replace_binary_for_update now routes Windows through stage_windows_replacement, which writes the staged binary + .cmd and actually executes it via a detached cmd.exe helper (CREATE_NO_WINDOW, stdio nulled) that retries move until the running process exits, optionally restarts the daemon (bsk daemon start) when the CLI path stopped one, self-deletes, and cleans up + errors visibly if the helper fails to spawn.
  • Non-Windows keeps the atomic write/rename path untouched.

Verification

• cargo fmt --all: clean (no changes)
• cargo test -p bsk-protocol: 114 passed
• cargo test -p bsk --lib: 228 passed, 1 failed — skill_install::sync::tests::sync_continues_on_partial_error, a pre-existing environment failure that fails identically on origin/main (unrelated to this change)
• cargo clippy -p bsk-protocol -p bsk --all-targets -- -D warnings: clean
• Windows-only code is #[cfg(windows)]-gated (imports included); a full x86_64-pc-windows-msvc check was not runnable in this Linux environment (aws-lc-sys needs an MSVC C toolchain).