fix: only write vcpkg cache on dependency and toolset changes

[Copilot]

The pipeline was keyed on github.run_id, guaranteeing a cache miss every restore and an unconditional new entry every save — even when vcpkg binaries were identical.

Changes

• Precise dependency hash — a Compute vcpkg cache hash step uses jq to extract only features and dependencies from vcpkg.json (excluding the app version field) and combines that with vcpkg-configuration.json before hashing. This prevents a version-only bump in vcpkg.json from needlessly invalidating the cache.
• vcpkg ref in cache key — VCPKG_REF is defined as a workflow-level env var and embedded in every cache key. Bumping the vcpkg toolset ref now correctly busts the cache, and the same value drives both the Checkout vcpkg step and the cache key, keeping them in sync.
• Conditional save — each save step runs only when cache-hit != 'true' on its paired restore step, skipping the write entirely on an exact hit.

env:
  VCPKG_REF: '2026.01.16'

- name: Compute vcpkg cache hash
  id: vcpkg-hash
  shell: bash
  run: |
    hash=$(jq -cj '{features,dependencies}' WebFrame/vcpkg.json | cat - WebFrame/vcpkg-configuration.json | sha256sum | awk '{print $1}')
    echo "value=${hash}" >> $GITHUB_OUTPUT

- name: "Restore VCPKG cache"
  id: restore-test-cache
  uses: actions/cache/restore@v4
  with:
    path: ${{ github.workspace }}/vcpkg-test-cache
    key: vcpkg-test-cache-${{ env.VCPKG_REF }}-${{ steps.vcpkg-hash.outputs.value }}
    restore-keys: vcpkg-test-cache-${{ env.VCPKG_REF }}

- name: "Save VCPKG cache"
  if: steps.restore-test-cache.outputs.cache-hit != 'true'
  uses: actions/cache/save@v4
  with:
    path: ${{ github.workspace }}/vcpkg-test-cache
    key: vcpkg-test-cache-${{ env.VCPKG_REF }}-${{ steps.vcpkg-hash.outputs.value }}

The same pattern is applied to the matrix build job (restore-build-cache / actions/cache/restore@v5).

Original prompt

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This section details on the original issue you should resolve

<issue_title>Cache Usage</issue_title>
<issue_description>The CI/CD pipeline should only save a new cache entry if the vcpkg binary contents have changed. Update the steps in main.yml to only overwrite stale caches.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Cache Usage #15

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