Start install downloads before local-only checks

[MikeMcQuaid]

• brew install only started network transfers after preinstall checks and, in ask mode, fetched each bottle manifest serially while computing the dry-run plan.
• Enqueue bottle manifests on the shared download queue right after building formula installers in both ask and no-ask modes so transfers overlap preinstall checks and dependant scanning and manifests for multiple formulae download concurrently. Ask mode drains them under a Fetching bottle manifests heading before printing the plan; warm runs enqueue nothing and stay silent.
• Once downloads are confirmed (--yes installs, reinstall, upgrade and other Install.fetch_formulae callers), also enqueue the formula's own bottle in FormulaInstaller#prelude_fetch: the blob URL needs neither the manifest nor dependency resolution, so both transfer concurrently and staging joins the same queue cycle.
• Skip the enqueue_fetch requeue for bottles the prelude fetch already enqueued so a completed early download is not reported a second time.
• Instrument Install.perform_preinstall_checks_once as a preinstall_checks phase timing to keep the reordering visible.
• Archive-cold brew install hello: the first transfer starts at ~375ms instead of ~442ms, the bottle no longer waits for the manifest round trip (~672ms before, ~375ms now) and --yes wall time drops around 30%. Two-formula ask-mode installs drop one full manifest round trip (~1240ms to ~1030ms).

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• I did not use AI/LLM to create this PR, or I disclosed the tool/model below and reviewed its output; I did not attribute commits to AI and will answer maintainer questions and review comments myself without AI/LLM.

Claude Fable 5 max with local review and testing.

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[Copilot]

Pull request overview

This PR optimises brew install by overlapping network transfers (bottle manifests and, once confirmed, bottles) with local-only preinstall work, improving wall-clock install time and making the reordering visible via phase timing instrumentation.

Changes:

• Start enqueuing bottle manifest downloads as soon as formula installers are constructed (including ask-mode), so manifest downloads run concurrently.
• For confirmed installs, enqueue the formula’s bottle download earlier in FormulaInstaller#prelude_fetch and avoid requeueing it later in enqueue_fetch.
• Add a dedicated preinstall_checks phase timing and expand/adjust unit tests to cover the new queueing behaviour.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
Library/Homebrew/utils/phase_timings.rb	Instruments perform_preinstall_checks_once as a separate preinstall_checks phase.
Library/Homebrew/test/formula_installer_spec.rb	Adds coverage for early bottle enqueueing and ensures enqueue_fetch doesn’t requeue an already-enqueued bottle.
Library/Homebrew/test/cmd/install_spec.rb	Updates expectations for the new prelude fetch behaviour and adds an ask-mode queue drain test.
Library/Homebrew/install.rb	Adds a metadata_only path to prelude_fetch_formulae to support ask-mode manifest-only prefetching.
Library/Homebrew/formula_installer.rb	Splits metadata-only vs full prelude fetch work; enqueues bottle early and avoids duplicate enqueue/reporting later.
Library/Homebrew/cmd/install.rb	Reorders flow so early prefetch can overlap local checks; drains manifest downloads before printing the ask-mode plan.

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[SMillerDev]

@MikeMcQuaid could it be that this caused the failure in https://github.com/Homebrew/homebrew-core/actions/runs/30799099950/job/91641096768?pr=296731?