feat(core-testing): git-serving fixture helpers

serveFixtureRepo(fixtureDir) copies a fixture into a temp dir, commits
it as a fresh single-commit repo, bare-clones it, and serves it over
`git daemon --export-all` on an ephemeral localhost port (default) or
as a file:// URL (fallback for daemon-less environments). Returns
{ cloneUrl, bareRepoPath, protocol, stop } with idempotent teardown.
Tests exercise a real `git clone` of fixtures/vite-kitchen over both
protocols. New subpath export @repo/core-testing/git — node-only, so
deliberately not on the jsdom root barrel. No new runtime deps: node
built-ins + system git (present in git's exec-path on macOS and the
ubuntu CI image).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016j8z4VHjedXDTjEDNg7qHK

packages/core-testing/package.json

@@ -7,6 +7,7 @@
     ".": "./src/index.ts",
     "./factory": "./src/factory/index.ts",
     "./contract": "./src/contract/index.ts",
+    "./git": "./src/git/index.ts",
     "./instrumentation": "./src/instrumentation/index.ts",
     "./react": "./src/react/index.ts",
     "./payload": "./src/payload/index.ts",