Escape 'type' keyword in generated Rust code

Use raw identifiers for fields named "type" to avoid conflicts with the
Rust keyword. Fix field number indexing in tests.

AGENTS.md

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+## Ask for clarification
+
+Unless the request is extremely clear, assemble a list of questions up front. After you begin work,
+you should be able to work without user assistance.
+
+## Coding
+
+If you are writing code, write tests first. The tests must pass for your work to be complete.
+
+## Special instructions
+
+### Fork
+
+If the users asks you to fork off and work on something, this means that you should:
+
+1. Create a temporary directory using `mktemp -d`
+2. Create a new branch to work on
+3. Use git worktree to extract that branch to the temporary directory (i.e., `git worktree add $TMP_DIR -- $BRANCH)
+4. Work from that directory (i.e., `cd $TMP_DIR`)