Record startup phases in brew prof --timings

[MikeMcQuaid]

• startup ended after require "global" and the next event was api_metadata_load, hiding ~130ms of a ~260ms warm no-op install
• command_load covers Commands.valid_internal_cmd?, which requires the command's file and so pays for its whole require graph: at ~90-135ms this is the single largest phase of brew install
• cli_parse covers both the top-level parse and cmd_class.new, the latter being where named args inflate formulae
• measure now no-ops until start! runs, so the startup path can measure unconditionally; require "json" moves into write!

This will help with future performance optimisation work.

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Claude Opus 5 high with local review and testing.

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[Copilot]

Pull request overview

This PR improves brew prof --timings phase visibility during early startup by making phase measurement safe to call unconditionally and by adding new startup-focused timing phases to brew.rb, enabling more actionable performance breakdowns.

Changes:

• Make Homebrew::PhaseTimings.measure a no-op until start! has enabled recording, and defer require "json" until write!.
• Add cli_parse and command_load timing measurements in Library/Homebrew/brew.rb to capture CLI parsing and command require-graph loading time.
• Update brew prof --timings integration expectations to include the new phases.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
Library/Homebrew/utils/phase_timings.rb	Make measurement opt-in and defer JSON loading until writing output.
Library/Homebrew/brew.rb	Add new startup-phase timing measurements (cli_parse, command_load).
Library/Homebrew/test/dev-cmd/prof_spec.rb	Adjust integration test to expect the new timing phases.

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