Windows: buzz_managed_node_bin_dir has no Windows arm — harness adapter install fails with "no managed Node.js runtime" #2491

Description

The one-click "Install" flow for agent harness adapters cannot work on Windows:
the managed Node.js runtime has no Windows support, so
managed_node_runtime_supported() is always false and the install fails with:

Buzz does not provide a managed Node.js runtime for windows-x86_64 yet

The hint says "Install Node.js from https://nodejs.org, restart Buzz, then
click Install again", but the managed-runtime path is still taken and still
fails, so a fresh Windows user cannot get the ACP adapters installed from the
UI at all.

Where Windows support is missing

1. desktop/src-tauri/src/managed_agents/managed_node_paths.rs —
   buzz_managed_node_bin_dir() matches only macOS/Linux (os, arch) pairs
   and returns None for ("windows", _). (Notably buzz_managed_npm_bin_dir()
   in the same file does have a #[cfg(windows)] arm, so Windows is halfway
   considered already.)
2. Same file — buzz_managed_node_bin_path() joins "node", which on Windows
   needs to be node.exe.
3. desktop/src-tauri/src/commands/agent_discovery/managed_node.rs — the
   MANAGED_NODE_ARTIFACT cfg chain covers darwin-arm64/x64 and
   linux-x64/arm64 only; the fallback is None. Node.js does publish Windows
   artifacts for the pinned version (node-v24.11.0-win-x64.zip), but note the
   Windows artifact is a .zip, not a .tar.gz, so extraction needs a second
   code path.

Suggested fix

Add a win-x64 (and optionally win-arm64) arm to all three sites: platform
mapping in buzz_managed_node_bin_dir, .exe-suffixed binary path, and a
MANAGED_NODE_ARTIFACT entry with zip extraction for the Windows artifact.

Workaround I use today

npm install --prefix %APPDATA%\Buzz\node-tools the two
@agentclientprotocol/*-acp packages by hand and copy a node.exe next to the
shims — after which the adapters are discovered and run fine, so this is purely
an install-path gap.

Environment

• Windows 11 Pro 25H2 (build 26200)
• Buzz desktop v0.4.23, built from source (MSVC, Rust 1.95.0)