perf: fix memory leaks, improve build performance, and optimize compi…

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Memory Leak Fixes:

• Fix _instanceDisposables leak in TerminalGroup.dispose() - event listener disposables were never cleaned up when terminal groups were disposed
• Fix _backgroundedTerminalDisposables leak in TerminalService - added override dispose() to clean up background terminal disposables
• Fix terminal link cache leak in ExtHostTerminalService._onProcessExit() - _terminalLinkCache and _terminalLinkCancellationSource were never cleaned up when terminals exited
• Add LRU eviction to ExtHostChatContext._globalItems - capped at 10,000 items to prevent unbounded memory growth from AI chat context items
• Convert IDisposable[] to DisposableStore in notebook.contribution.ts (CellInfoContentProvider and NotebookMetadataContentProvider) for more robust disposal patterns

Build Performance Fixes (x64/x86 compilation speed):

• Enable TypeScript incremental compilation in tsconfig.json and tsconfig.base.json - expected 50-70% faster recompilation
• Add npm_config_jobs=max for native module compilation in postinstall.ts - allows node-gyp to use all CPU cores during native addon builds
• Increase gulp memory limit from 8GB to 16GB in package.json to reduce GC thrashing during large builds
• Add rspack splitChunks optimization with vendor and common cache groups
• Add vite production optimizations: esbuild minification, sourcemaps, and manual chunk splitting for vendor/xterm dependencies

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