Migrate libdav1d build pipeline to BuildShared and tighten workflow token permissions

[Copilot]

This updates libdav1d-build to the shared MPVKit build/release flow and completes the migration by removing repo-local orchestration logic. It also resolves workflow security findings by explicitly scoping GITHUB_TOKEN permissions for reusable workflow jobs.

• Build script migration to BuildShared

  • Sources/BuildScripts/Package.swift
    • adds executable product declaration
    • adds BuildShared dependency and target linkage
    • sets macOS platform metadata consistent with shared build scripts
  • Sources/BuildScripts/XCFrameworkBuild/main.swift
    • replaces local command parsing/execution path with BuildRunner.performCommand()
    • adopts BuildLibrary contract for Library
    • updates BuildDav1d init path to pass ArgumentOptions into BaseBuild
    • removes force-unwrap behavior in target URL generation and uses explicit precondition messaging for missing releaseVersion

• Workflow consolidation to reusable BuildShared workflows

  • .github/workflows/build.yml
    • replaces bespoke build/release steps with mpvkit/BuildShared/.github/workflows/build.yml@main
    • passes library: libdav1d and dispatch version input
  • .github/workflows/check_version.yml
    • replaces custom version-check/PR logic with mpvkit/BuildShared/.github/workflows/check_version.yml@main
    • passes upstream source repo (videolan/dav1d) and library identifier

• Security hardening for Actions token scope

  • adds explicit job-level permissions for reusable workflow invocations:
    • build job: contents: write
    • check_version job: contents: write, pull-requests: write

• Repository hygiene/documentation alignment

  • removes obsolete local build implementation file:
    • Sources/BuildScripts/XCFrameworkBuild/base.swift
  • appends Sources/BuildScripts/Package.resolved to .gitignore
  • aligns README/docs template formatting/content with current MPVKit build-repo conventions

// Sources/BuildScripts/XCFrameworkBuild/main.swift
do {
    let options = try BuildRunner.performCommand()
    try BuildDav1d(options: options).buildALL()
} catch {
    print(error.localizedDescription)
    exit(1)
}