sx-git Phase 2: refs — branches/tags/HEAD over persist kv, CAS + reflog (TDD)

Ref value = {:cid} | {:symref}; atomic moves via persist/kv-cas old-value
expect, create-only branches via kv-put-new; bounded symref resolution;
per-ref append-only reflog on the persist log facet. 38/38, total 76/76.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/git/scoreboard.md

@@ -5,4 +5,5 @@ _Generated by `lib/git/conformance.sh`_
 | Suite | Pass | Fail | Total |
 |-------|-----:|-----:|------:|
 | object | 38 | 0 | 38 |
-| **Total** | **38** | **0** | **38** |
+| ref | 38 | 0 | 38 |
+| **Total** | **76** | **0** | **76** |