feat: rendering library v1 (#15778)

The goal will be to replace askama

codex-rs/utils/template/README.md

@@ -0,0 +1,41 @@
+# codex-utils-template
+
+Small, strict string templating for prompt and text assets.
+
+Supported syntax:
+
+- `{{ name }}` placeholder interpolation
+- `{{{{` for a literal `{{`
+- `}}}}` for a literal `}}`
+
+The library is intentionally strict:
+
+- parsing fails on malformed placeholders
+- rendering fails on missing values
+- rendering fails on duplicate values
+- rendering fails on extra values not used by the template
+
+## Example
+
+```rust
+use codex_utils_template::Template;
+use codex_utils_template::render;
+
+let template = Template::parse(
+    "Hello, {{ name }}.\nLiteral braces: {{{{ and }}}}.\nMode: {{ mode }}",
+)?;
+
+let rendered = template.render([
+    ("name", "Codex"),
+    ("mode", "strict"),
+])?;
+
+assert_eq!(
+    rendered,
+    "Hello, Codex.\nLiteral braces: {{ and }}.\nMode: strict"
+);
+
+let one_shot = render("Hi {{ who }}!", [("who", "there")])?;
+assert_eq!(one_shot, "Hi there!");
+# Ok::<(), Box<dyn std::error::Error>>(())
+```