Task: Fix vcpkg caching (x-gha removed upstream)

This page documents a task in the Engineering hygiene story. It captures the goal, current status, acceptance, and any notes or results.

1. Goal

Restore vcpkg binary caching across all five CI workflows after the x-gha backend was silently removed upstream.

x-gha backend replaced with files provider backed by actions/cache@v4.
All 5 workflows updated: canary-linux, continuous-linux, continuous-windows, continuous-macos, nightly-linux.
First run populates the cache; subsequent runs drop vcpkg install from ~21 min to seconds.

Captured during execution; cleared into the parent story on close.

Silent upstream removal meant every CI run was rebuilding 120 packages from source. The cache hit-rate is the load-bearing piece of CI speed.

CI back to expected times; no more 21-min vcpkg installs.