Refactor libshaderc build pipeline to BuildShared and align package/docs metadata

[Copilot]

This change aligns libshaderc-build with the shared mpvkit/BuildShared architecture used by other build repos (e.g. lcms2-build), including workflow reuse and script-layer consolidation. It also updates package template/docs metadata and appends the required BuildScripts lockfile ignore entry.

• Build script architecture (BuildShared adoption)

  • Sources/BuildScripts/Package.swift now depends on https://github.com/mpvkit/BuildShared.git (main) and links target build against BuildShared.
  • Sources/BuildScripts/XCFrameworkBuild/main.swift migrated from local runner/base abstractions to BuildRunner/BaseBuild from BuildShared.
  • Removed local duplicated build core: Sources/BuildScripts/XCFrameworkBuild/base.swift.
  • Replaced release URL version sourcing from local static state with explicit runtime propagation from parsed options to avoid force-unwrapped global access.

• GitHub Actions workflow refactor

  • Replaced inline job implementations with reusable workflows:
    • .github/workflows/build.yml → mpvkit/BuildShared/.github/workflows/build.yml@main
    • .github/workflows/check_version.yml → mpvkit/BuildShared/.github/workflows/check_version.yml@main
  • Passed repo-specific inputs (library: libshaderc, repository: google/shaderc, dispatch version passthrough).
  • Added explicit permissions blocks in both workflow jobs.

• Package template + documentation updates

  • Updated docs/Package.template.swift product export to point to the dummy embedding target (_Libshaderc_combined) while preserving auto-generation markers/structure.
  • Updated README.md with MPVKit component context line, consistent with sibling build repositories.

• Repository hygiene

  • Appended Sources/BuildScripts/Package.resolved to the end of .gitignore (as requested).

Example of the workflow consolidation pattern applied:

jobs:
  build:
    permissions:
      contents: write
    uses: mpvkit/BuildShared/.github/workflows/build.yml@main
    secrets: inherit
    with:
      version: '${{ github.event.inputs.version }}'
      library: 'libshaderc'