docs: publish Tend Compose continuity evidence

Export the reviewed allowlisted snapshot from private source commit 07c1655921f21ee5e4fc4d85639d199e8867b17d. This records the Docker Compose activation, schema-compatible rollback, and stateful migration resource findings from Observatory Preview 19 dogfooding.

AI-Assisted: OpenAI Codex
Signed-off-by: Cole Speelman <crspeelman@gmail.com>

docs/PUBLIC_SNAPSHOT.md

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+# Public snapshot boundary
+
+Private development happens in `gamertan/tend-dev`. The canonical public
+repository is not a mirror of that Git history. `scripts/export-public.sh`
+creates an exact-file, exact-commit, allowlisted filesystem snapshot and records
+its source commit and tree in `PUBLIC-SNAPSHOT.json`.
+
+The exporter refuses dirty or unpushed source, destinations inside the source
+or Git metadata, workflow directories, non-allowlisted paths, and known private
+material markers. The resulting directory receives a new public root commit.
+
+Public Gitea is canonical for issues, contributions, and releases. GitHub is a
+read-only discovery copy of the same public tree. Tags belong only to canonical
+Gitea.
+
+The snapshot includes the program, security and architecture documentation,
+copyable examples, release configuration, and local verification scripts.
+Private workflows, runner configuration, repository credentials, and raw
+operational evidence remain outside the public root.