Avoid duplicate cache traversal for relative lists

[p-x9]

Summary

• add relativeListLocation(for:) to resolve the Mach-O image, backing dyld cache, and file offset together
• use the resolved backing cache when reading relative method, property, and protocol list headers
• avoid repeating cache/subcache traversal through separate cacheAndFileOffset and _machO(at:) lookups

Why

Relative list-list entries need three related values: the image selected by imageIndex, the cache file that backs that image, and the list's file offset. Resolving these independently caused overlapping cache/subcache scans at each method, property, and protocol list call site.

The new helper keeps those values associated and performs the lookup once. The returned file offset is intentionally read through the located cache's file handle; it is not assumed to match the image's own file handle.

Benchmark

Compared clean worktrees of main and this commit with:

DyldCache.MachOFile.objc.classInfo.withMethodRelativeList.first100

Median results:

Metric	main	This PR	Change
Wall time	65.2 ms	59.9 ms	8% faster
CPU time	64.6 ms	59.9 ms	7% faster
Throughput	15/s	17/s	13% higher
Peak RSS	40 MB	39 MB	2% lower

Instruction count was effectively unchanged (1169M to 1167M).

Validation

• git diff --check main...HEAD
• focused benchmark comparison against a freshly generated main baseline

[Copilot]

Pull request overview

This PR optimizes dyld shared cache relative list-list parsing by introducing a single helper that resolves the target Mach-O image, its backing cache, and the correct cache-relative file offset together, reducing repeated cache/subcache traversal at each call site.

Changes:

• Add MachOFile.relativeListLocation(for:) to resolve (image, backing cache, file offset) in one step.
• Update method/property/protocol relative list-list readers to use the resolved backing cache’s file handle for header reads.
• Remove duplicated cacheAndFileOffset(fromStart:) + _machO(at:) lookups at the call sites.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
Sources/MachOObjCSection/Model/Protocol/ObjCProtocolRelativeListList.swift	Switch protocol relative list-list header reads to use relativeListLocation(for:) and return the resolved image.
Sources/MachOObjCSection/Model/Property/ObjCPropertyRelativeListList.swift	Use relativeListLocation(for:) for backing-cache header reads and is64Bit from the resolved image.
Sources/MachOObjCSection/Model/Method/ObjCMethodRelativeListList.swift	Use relativeListLocation(for:) for backing-cache header reads and is64Bit from the resolved image.
Sources/MachOObjCSection/Extension/MachOFile+.swift	Introduce relativeListLocation(for:) helper to resolve image/cache/fileOffset together for relative list-list entries.

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