Implement typed template library and rendering

docs/USER_GUIDE.md

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+# Templates User Guide
+
+Open **Templates** to create or select a reusable definition.
+
+1. Choose the template type and the contexts in which it may be used, such as
+   `campaign.postal`.
+2. Declare every required input path and its type. Use the same paths as tokens
+   in the body, for example `{{name}}` or `{{postal.address}}`.
+3. Configure page size and, for label sheets, rows, columns, and spacing.
+4. Save to create a new immutable revision. Publish the revision before using
+   it for final output.
+5. In **Preview**, supply a representative JSON item. Compatibility validation
+   explains missing fields, wrong types, unsupported usages, and output-format
+   mismatches before output is produced.
+6. Preview a draft or render final output. Store it in Files when that module is
+   available and you have upload permission; otherwise use the bounded download.
+
+Render evidence shows the exact revision and abbreviated template, input, and
+output hashes. A consumer such as Campaign can submit many frozen recipients;
+the UI sample intentionally validates one representative item.