Filter Linux cask loads by support

[MikeMcQuaid]

• Keep brew audit --tap and add brew readall loading Linux-supported casks so Linux syntax errors still surface.
• Skip macOS-only casks before Linux reloads because their macOS sha256 arm:/intel: shorthand is invalid in a Linux context.

Fixes #22148.

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OpenAI Codex 5.5 with local review and tweaks

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[Copilot]

Pull request overview

This PR fixes Linux failures in tap syntax checks by avoiding eager loading of macOS-only casks in a Linux context (where macOS-only sha256 arm:/intel: shorthand can be invalid), while still ensuring Linux-supported casks are loaded on Linux so Linux-specific syntax errors are detected.

Changes:

• Update brew audit --tap to filter cask loading via a helper that skips macOS-only casks when auditing on Linux.
• Implement Linux-specific cask validation in Readall.valid_casks? by first identifying Linux-supported casks (under a macOS simulation) and then loading those casks under a Linux simulation.
• Add/adjust tests covering the new Linux tap cask-loading behavior for both audit and readall.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
Library/Homebrew/dev-cmd/audit.rb	Adds cask_for_audit and uses it to avoid loading macOS-only casks on Linux during --tap/--changed collection.
Library/Homebrew/readall.rb	Implements Linux-only cask loading/validation by filtering for Linux-supported casks and then loading them under Linux simulation.
Library/Homebrew/extend/os/mac/readall.rb	Delegates Linux validation to the base Readall.valid_casks? implementation when os_name == :linux.
Library/Homebrew/test/dev-cmd/audit_spec.rb	Adds a regression test to ensure brew audit --tap on Linux skips macOS-only casks but still loads Linux-supported casks.
Library/Homebrew/test/cmd/readall_spec.rb	Adds a test asserting Linux readall output excludes macOS-only casks while still reporting Linux-supported cask failures.

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