Make text-view template substitution single-pass

[rickardvh]

Summary

• make Python output.emit text-view template substitution single-pass over the original template, matching TypeScript behavior
• preserve placeholder-looking replacement text literally instead of expanding it again
• document the single-pass substitution rule

Blocker Coverage

Addresses the remaining post-merge #81 portability blocker:

{
  "result": {"first": "{second}", "second": "resolved"},
  "template": "{first} {second}"
}

Python now renders {second} resolved, matching TypeScript instead of recursively producing resolved resolved.

Validation

• uv run pytest tests/test_public_api.py::test_generated_output_emit_text_views_execute_in_python_and_typescript -q
• uv run ruff check src/command_generation/primitive_executor.py tests/test_public_api.py
• git diff --check origin/master...HEAD
• uv run pytest

Follow-up to #81.