docs(faq): Add FAQ section and class methods explanation

docs/faq/class_methods.md

@@ -0,0 +1,75 @@
+# Calling Python Class Methods Inside a Kernel
+
+## The problem
+
+If you try to call a standard Python class method, such as
+`self.get_value()`, from inside a function decorated with
+`@basic` or `@structural`, the compiler will fail. Historically, before Kirin
+v0.16.8, this could appear as an `AttributeError` about `arg_names`.
+
+## Why this happens
+
+A kernel is a piece of code that describes instructions intended to run on the
+target device, not on your host machine. Because of this, the Kirin compiler
+cannot evaluate arbitrary Python runtime expressions, such as object attribute
+lookups or dynamically calling class methods, during device execution.
+
+The compiler only parses global constants and already-resolved kernel functions.
+
+## The solution
+
+Resolve the Python method on the host before defining the kernel. By assigning
+the result to a local variable, the inner kernel can cleanly capture that
+variable as a constant.
+
+### Incorrect
+
+This tries to evaluate the method inside the device kernel.
+
+```python
+from kirin.prelude import basic
+
+
+class Offset:
+    def __init__(self, value: int):
+        self.value = value
+
+    def get_value(self) -> int:
+        return self.value
+
+    def bad_method(self):
+        @basic
+        def add_offset(x: int) -> int:
+            # ERROR: Kirin cannot evaluate arbitrary Python class methods.
+            return x + self.get_value()
+
+        return add_offset
+```
+
+### Correct
+
+Resolve the method on the host side, then capture it in the kernel closure.
+
+```python
+from kirin.prelude import basic
+
+
+class Offset:
+    def __init__(self, value: int):
+        self.value = value
+
+    def get_value(self) -> int:
+        return self.value
+
+    def method(self):
+        # 1. Resolve the method in host Python code.
+        offset = self.get_value()
+
+        # 2. Define the device kernel.
+        @basic
+        def add_offset(x: int) -> int:
+            # 3. Use the captured constant inside the kernel.
+            return x + offset
+
+        return add_offset
+```

docs/faq/index.md

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+# FAQ
+
+This page collects common questions with a short answer and a link to a deeper explanation.
+
+## Kernel authoring
+
+| Question | Short answer |
+| --- | --- |
+| [Why do I get an `AttributeError` when calling a class method inside a kernel?](class_methods.md) | Resolve the Python method on the host side before defining the kernel, then capture it as a constant. |

mkdocs.yml

@@ -26,6 +26,9 @@ nav:
         - Structural Control Flow: dialects/scf.md
         - Immutable List: dialects/ilist.md
       - Compiler 101: 101.md
+      - FAQ:
+        - faq/index.md
+        - Class methods in kernels: faq/class_methods.md
       # - Comparison: comparison.md
       - Contributing: contrib.md
   - Cookbook: