Refactor libbluray-build to BuildShared and migrate workflows to reusable actions

[Copilot]

This PR aligns libbluray-build with the lcms2-build pattern by adopting mpvkit/BuildShared for build orchestration and moving CI logic to reusable workflows. It also updates package/docs metadata and repository ignore rules to match the new structure.

• BuildScripts refactor (BuildShared integration)

  • Replaced local monolithic build framework usage with BuildShared APIs.
  • Updated Sources/BuildScripts/Package.swift to add:
    • BuildShared dependency
    • executable product declaration
    • target dependency on BuildShared
    • macOS platform constraint (.macOS(.v11))
  • Refactored Sources/BuildScripts/XCFrameworkBuild/main.swift to use:
    • BuildRunner.performCommand()
    • BuildLibrary-conforming Library enum
    • BaseBuild / ZipBaseBuild from BuildShared
  • Removed Sources/BuildScripts/XCFrameworkBuild/base.swift (local duplicated framework implementation).

• GitHub Actions migration

  • Replaced custom workflow implementations with reusable workflows from mpvkit/BuildShared:
    • .github/workflows/build.yml → BuildShared/.github/workflows/build.yml@main
    • .github/workflows/check_version.yml → BuildShared/.github/workflows/check_version.yml@main
  • Wired libbluray-specific inputs (library, repository, custom pr_body) in the delegating workflows.
  • Kept explicit job-level permissions blocks in local workflow files.

• Docs/template alignment

  • Updated docs/Package.template.swift platform baselines to .macOS(.v11), .iOS(.v14), .tvOS(.v14) and normalized product formatting.
  • Updated README.md to include MPVKit component context while preserving build usage instructions.

• Repository hygiene

  • Appended Sources/BuildScripts/Package.resolved to the end of .gitignore.

// Sources/BuildScripts/XCFrameworkBuild/main.swift
import Foundation
import BuildShared

let options = try BuildRunner.performCommand()
try BuildOpenSSL(options: options).buildALL()
try BuildFreetype(options: options).buildALL()
try BuildBluray(options: options).buildALL()