Add pure-Ruby ad-hoc signing

[MikeMcQuaid]

• remove the /usr/bin/codesign dependency from MachO.codesign! so Homebrew can repair bottle signatures on Apple Silicon using Ruby
• parse and build XNU-compatible SuperBlob and CodeDirectory data so replacement signatures can retain security-relevant metadata
• add or replace LC_CODE_SIGNATURE, resize __LINKEDIT and rebuild fat offsets so every page hash describes the final on-disk layout
• use SHA-256 with legacy SHA-1 agility only for old deployment targets because XNU accepts the open ld64 ad-hoc format without an identity
• preserve non-linker requirements, entitlements, flags and runtime metadata while deliberately replacing linker-generated metadata
• validate before an in-place write to retain hard links and leave failed inputs unchanged
• cover thin, fat, byte-order, metadata and macOS verification paths with regression tests, and document the compatibility boundaries

Fixes #262

[Copilot]

Pull request overview

This PR adds a pure-Ruby implementation of ad-hoc code signing for Mach-O binaries, removing the runtime dependency on invoking /usr/bin/codesign and enabling signature repair after Mach-O modifications (notably for Homebrew bottle relocation on Apple Silicon).

Changes:

• Introduces a new MachO::CodeSigning implementation that can build and embed XNU-compatible SuperBlob/CodeDirectory signatures (with SHA-256 and conditional SHA-1 agility).
• Extends MachOFile and FatFile with #codesign!, updating LC_CODE_SIGNATURE, resizing __LINKEDIT, and rebuilding fat slice offsets/sizes as needed.
• Adds regression tests and documentation for ad-hoc signing behavior and compatibility boundaries.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
test/test_code_signing.rb	Adds regression coverage for unsigned/signed, fat/thin, metadata preservation, and failure-safety behaviors.
README.md	Documents the new pure-Ruby ad-hoc signing behavior, constraints, and scope.
lib/macho/macho_file.rb	Adds MachOFile#codesign! and improves low_fileoff handling for segments without sections.
lib/macho/load_commands.rb	Makes LC_CODE_SIGNATURE modifiable; adds serialization for LinkeditDataCommand and parsing helper for embedded signatures.
lib/macho/fat_file.rb	Adds FatFile#codesign! and a rebuild path for resized slices with updated offsets/sizes.
lib/macho/code_signing.rb	Implements code-signing structures (Blob/SuperBlob/CodeDirectory) and the ad-hoc signer logic.
lib/macho.rb	Switches MachO.codesign! to use the in-library implementation and normalizes error handling.

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[MikeMcQuaid]

Working as expected from Homebrew/brew#23047