feat: add comprehensive FileSystem observer API types with full MDN spec compliance

[Copilot]

Added complete TypeScript type definitions for the File System Access API observer interfaces to src/fsa/types.ts, implementing the full MDN specification.

New Types Added

FileSystemChangeRecord

Represents a file system change event with comprehensive properties:

• changedHandle: IFileSystemHandle | IFileSystemSyncAccessHandle | null - The changed file system handle (null for "disappeared", "errored", or "unknown" types)
• relativePathComponents: string[] - Path components from the observed directory to the changed handle
• relativePathMovedFrom: string[] | null - Former location path for "moved" type events
• root: IFileSystemHandle | IFileSystemSyncAccessHandle - Reference to the root handle passed to observe()
• type: 'appeared' | 'disappeared' | 'modified' | 'moved' | 'errored' | 'unknown' - Complete set of change types

IFileSystemObserver

Observer interface with constructor signature and methods:

• Constructor with callback function (records, observer) => void
• observe(handle, options?) - Start observing changes to a directory handle
• unobserve(handle) - Stop observing changes to a specific directory handle
• disconnect() - Disconnect and stop all observations

FileSystemObserverObserveOptions

Options interface for the observe method:

• recursive?: boolean - Whether to observe changes recursively in subdirectories

Implementation Details

• Full compliance with MDN File System Access API specification
• Support for both user-observable file system and Origin Private File System (OPFS) with IFileSystemSyncAccessHandle
• Proper nullable types for handles based on event types
• Complete JSDoc documentation with MDN references
• Constructor typing enables proper instantiation: new FileSystemObserver(callback)

These types enable future implementation of file system watching functionality for the watchFile, unwatchFile, and watch methods in FsaNodeFs.

Fixes #1170.

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[streamich]

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