fix: install platform-correct native binaries for extensions (#102)

[jackson-zhou]

Summary

Fixes #102. Platform-specific extensions (e.g. Kilo Code, which bundles kilo/ffmpeg/bwrap native binaries) installed the wrong architecture on macOS - the downloaded bin/kilo was an ELF ... musl aarch64 (Alpine Linux) binary, causing ENOEXEC.

Root cause

The SideX marketplace proxy's search returns a single "latest" version whose targetPlatform field is omitted but whose VSIX asset URI (base64-encoded in the path) points at alpine-arm64. The proxy serves whatever platform is baked into that base64 and ignores the targetPlatform query parameter, so every OS downloaded the alpine-arm64 build.

Changes (6 files)

• marketplace.rs / lib.rs: add current_target_platform() that maps the current build target (Rust cfg!, always correct for the host) to Open VSX's targetPlatform format (e.g. darwin-arm64, linux-x64).
• extensions.rs: rewrite_proxy_vsix_platform decodes the proxy's base64 VSIX URL, swaps the embedded platform to current_target_platform(), and re-encodes. This is the authoritative correction point - the proxy can't be fixed without deploy access, so the backend rewrites the URL before downloading. ensure_target_platform also appends the param for non-proxy URLs. Includes unit tests.
• extensionResourceLoader.ts: use the query-param form (version?targetPlatform=) for Open VSX manifest URLs in Tauri. The version+platform path segment that the Microsoft Marketplace uses 404s on Open VSX, aborting installation before the backend download runs.
• host.cjs: stub vscode.window notebook/editor APIs (visibleNotebookEditors, activeNotebookEditor) in the SideX Node extension host.
• extHost.api.impl.ts: same notebook API stubs on the webview-side extension host, where kilo-code actually activates.

Why notebook API stubs are required for #102

The host.cjs / extHost.api.impl.ts changes are not a separate improvement - they are required for #102 to be verifiably fixed:

1. Extension installation lacks platform detection — native binaries (e.g. Kilo Code) download wrong architecture on macOS #102's core fix (platform detection + URL rewrite) makes SideX download the correct platform-specific VSIX (e.g. darwin-arm64 instead of alpine-arm64).
2. But after installing the correct VSIX, extensions with native binaries (the exact extensions Extension installation lacks platform detection — native binaries (e.g. Kilo Code) download wrong architecture on macOS #102 is about) call vscode.window.visibleNotebookEditors.map(...) during activate().
3. SideX's extension host hasn't implemented these notebook window APIs, so visibleNotebookEditors is undefined, and .map() throws "Cannot read properties of undefined" - the extension fails to activate, registers no commands, and the sidebar won't load.
4. Without the stubs, Extension installation lacks platform detection — native binaries (e.g. Kilo Code) download wrong architecture on macOS #102 is unfixable in practice: the correct binary downloads but the extension still can't run. The bug report ("can't install/use Kilo Code") remains unresolved.

The stubs are minimal (return empty array / undefined, matching VS Code's contract when no notebook editors are open) and only prevent the crash - they don't implement notebook functionality.

Verification (macOS arm64, dev mode)

• bin/kilo is Mach-O 64-bit executable arm64 (was ELF ... musl)
• Kilo Code sidebar loads, commands register, prompts send without ENOEXEC or "Cannot read properties of undefined"
• cargo check / eslint / prettier / cargo test (4 unit tests) all pass

Fixes #102