dyld cache accessors trap on truncated or malformed caches

[p-x9]

Several DyldCache / FullDyldCache accessors trap (try!, precondition, arithmetic overflow, numericCast) when header values are out of range, instead of returning nil. A viewer or tool that opens untrusted caches crashes before it can report anything.

Accessor	Location	Trap
DyldCache.init(url:)	DyldCache.swift:87	try! fileHandle.read(offset: 0) on a file shorter than dyld_cache_header
imageInfos, imageTextInfos	DyldCache.swift:223, :232	readDataSequence with the declared count; try! in swift-fileio-extra (_FileIOProtocol+.swift:36) when the array runs past EOF
slide info pageStarts / pageExtras / toc (v1–v5)	Model/DyldCache/SlideInfo/*	same readDataSequence path with untrusted page_starts_count, page_extras_count, toc_count
codeSign	DyldCache.swift:365	numericCast of codeSignatureSize to Int, then try! fileSlice(...)
expectedCacheFileSize	Protocol/DyldCacheRepresentable.swift:173	map.fileOffset + map.size + header.codeSignatureSize overflows
fileOffset(of:)	Protocol/DyldCacheRepresentable.swift:182	address - mapping.address + mapping.fileOffset can overflow with a malformed fileOffset

Reproduce

• A 16-byte file "dyld_v1 arm64e\0" → DyldCache(url:) traps.
• A copy of a real main cache file with any of these changes:
  • imagesCount = 0xFFFFFFFF → imageInfos traps.
  • imagesTextCount = UInt64.max / 2 → imageTextInfos traps.
  • codeSignatureSize = UInt64.max → codeSign and expectedCacheFileSize trap.

Suggestion

• Throw from init(url:) when the header cannot be read. It already throws.
• Validate offset + count * size against the file size (using checked arithmetic) before readDataSequence, and return nil otherwise. A throwing / optional readDataSequence in swift-fileio-extra would fix every caller at once.
• Use addingReportingOverflow for the range sums above.

Callers can currently only work around these by bounds-checking before each call.