fix: Route VST3 scanning through cache service and add native disk cache

[ChuxiJ]

Problem

VST3 scan cache is partially unused. Frontend scanner/cache services are tested, but app connection code can bypass them, and the native scanner cache is process-only while #1524 expects disk/native cache behavior.

Root Cause

Scanning was implemented in multiple layers during the bridge/native transition without one authoritative cache path.

Solution

• Route UI scan flows through the cache service consistently.
• Add native disk cache or document/defer the native SQLite requirement.
• Add invalidation rules for plugin install/remove/rescan.
• Add tests for cached load, forced rescan, and stale plugin removal.

Verification

• Run targeted VST3 scanner/cache tests.
• Add a native scan cache test that survives process restart or clearly documents in-memory behavior.
• Verify UI list_cached/rescan behavior with mock plugin paths.

Files to Touch

• src/hooks/useVST3Connection.ts
• src/services/vst3bridge/VST3PluginScanner.ts
• src/services/vst3bridge/VST3ScanCache.ts
• crates/ace-plugin-host/src/scanner.rs
• src-tauri/src/commands/plugin.rs