Add structured link install steps

[MikeMcQuaid]

Improved fix for Homebrew/homebrew-core#291532 and eliminates more post-install blocks.

• Let JSON API post-install data express PostgreSQL's versioned public links.
• Fix PostgreSQL post-install ordering by running link steps before initdb.
• Add link_dir for recursive links that keep real extension dirs, skip .DS_Store and replace conflicting files or symlinks.
• Add link_children for versioned executable links with implicit target paths and HOMEBREW_PREFIX as the link target default.
• Expand #{name}, #{version}, #{version.major} and #{version.major_minor} tokens from serialised step data.
• Normalise path, source and target values into typed path specs so runner methods do not need string-or-hash checks.
• Replace loose T.untyped install-step payloads with narrower aliases for raw steps, path specs, system arguments and template values.
• Keep Formula and Cask source or target base defaults intact while giving public link steps the prefix default needed by versioned formulae.

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[Copilot]

Pull request overview

This PR extends Homebrew’s declarative, JSON-serializable install steps to support structured “public link” operations (needed for versioned PostgreSQL installs), including new DSL methods and runner behavior, plus RuboCop and documentation updates.

Changes:

• Add link_dir (recursive) and link_children (direct children) install-step types, with support for {{name}} / {{version.*}} template tokens.
• Tighten and normalize serialized install-step payload typing and path-spec handling to avoid string-or-hash branching in the runner.
• Update RuboCop install-step validation + tests and document the new step DSL in the Formula Cookbook.

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
Library/Homebrew/install_steps.rb	Adds new step types (link_dir, link_children), template token support, and step normalization/type tightening.
Library/Homebrew/rubocops/shared/install_steps_helper.rb	Expands allowed DSL calls and permits limited template interpolation nodes.
Library/Homebrew/test/install_steps_spec.rb	Adds coverage for link steps running before initdb and .DS_Store skipping behavior.
Library/Homebrew/test/rubocops/install_steps_spec.rb	Updates RuboCop expectations for newly allowed DSL calls and template usage.
Library/Homebrew/test/formula_spec.rb	Simplifies ln_s usage now that defaults cover prior explicit bases.
Library/Homebrew/json_api_postinstall_preflight_postflight_plan.md	Updates planning doc to reflect new generic link steps for PostgreSQL.
docs/Formula-Cookbook.md	Documents link_dir and link_children DSL usage for formula authors.

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