Source history links

Purpose: Let readers open the exact committed bytes of a canonical record and the commit that introduced them. Scenario: /source/current/<path> serves the working-tree record; /source/blob/<commit>/<path> serves the record at a commit; /source/commits/<commit> lists a commit's files. Activity pages and record pages link to these. A retracted post's path returns the deletion notice at every identity. When Git no longer knows a commit because a history purge rewrote it, the routes resolve the identity through the history rewrite records and name the resolved commit in the response. Sources: controller (source unavailable), path and commit validators (source unavailable), commit map resolver (source unavailable), record spec. Known limitation: A commit that no record maps, such as one from a repository restored from backup, is simply not found.