- Switch schedule from nightly to every Sunday 03:00 UTC.
- Add resolve-upstream job that fetches the latest release tag from
  leejet/stable-diffusion.cpp via the GitHub API; all build jobs and
  the release job now check out that tag instead of the master tip.
- Release tag name now reuses the upstream tag name (drops local
  commit-count/hash computation). Existing-tag check makes repeated
  runs idempotent when upstream has no new release.

Asset names previously embedded the local CI commit short hash (from
github.sha via prompt/actions-commit-hash), which is decoupled from the
upstream source actually being built. Two consecutive releases of the
same upstream version therefore produced identical asset hashes, while
the release tag (taken from upstream) differed - making assets
untraceable to their source version.

Drive all artifact naming from the upstream release tag resolved by
the resolve-upstream job, so asset names match the release tag exactly
(e.g. sd-master-737-3b6c9ca-...). Drop the now-unused Get commit hash
steps and the top-level BRANCH_NAME env.

The previous commit left an empty `env:` mapping (the BRANCH_NAME entry
was removed but the key and its comment remained). GitHub's workflow
parser rejects an env mapping with no entries ("Unexpected value"), so
the workflow file was invalid. Drop the whole block - no job references
the top-level env.

Add an optional workflow_dispatch input `upstream_ref`. When set, the
resolve-upstream job uses that value as the tag to build and validates
it resolves to an existing upstream tag of leejet/stable-diffusion.cpp
(failing early with a clear error on a typo instead of failing every
build job). When empty, behavior is unchanged: build the latest
upstream release. Useful for rebuilding a specific historical version.

This repo is a fork of leejet/stable-diffusion.cpp and inherits every
upstream git tag, so the previous "git ls-remote --tags" check always
found the tag and skipped release creation — no release was ever
published. Gate on the GitHub Release itself via the
releases/tags/{tag_name} API instead, since that is the object we
actually create and it is not carried over by the fork.

Match windows-latest-cmake, which already sets
-DCMAKE_CXX_FLAGS='/bigobj'. Without it, recent upstream source
hits MSVC's "fatal error C1128: number of sections exceeded object
file format limit" on src/stable-diffusion.cpp.