Implement remoteFetchSymbols service client for local symbol caching #700

Summary

Implement a client for com.apple.dt.remoteFetchSymbols to download the dyld shared cache from iOS devices. This would enable local symbol resolution without needing Xcode.

Motivation

Currently, resolving symbol addresses (e.g. dlsym, objc_msgSend) in a remote process requires parsing Mach-O headers over GDB RSP memory reads (#699). While this works, having the shared cache locally would allow:

• Faster symbol resolution — parse locally instead of over USB/network
• Crash symbolication — resolve addresses in crash logs
• Offline analysis — inspect device libraries without being connected
• Cache per iOS version — download once, reuse for all devices on the same version

Currently only Xcode downloads these symbols (to ~/Library/Developer/Xcode/iOS DeviceSupport/). go-ios doesn't have this capability.

Implementation

The service is straightforward (see pymobiledevice3's RemoteFetchSymbolsService for reference):

1. Connect to com.apple.dt.remoteFetchSymbols via RemoteXPC
2. Send {"XPCDictionary_sideChannel": <uuid>, "DSCFilePaths": []} → receive file count
3. For each file: receive metadata (path, size), then stream chunks via file transfer
4. Store locally, indexed by iOS version/build

The shared cache is large (several GB), so downloading should be optional and cached per iOS version + build number.

Related

• Add debugviewhierarchy: capture iOS view hierarchy via pure GDB RSP #699 (debugviewhierarchy) would benefit from this for faster dlsym bootstrapping
• Similar to what Xcode does when you first connect a device ("Preparing debugger support")