Make Python API the source of truth for upstream coverage checks

Functions exposed in Python (e.g., as aliases of other Rust bindings)
were being falsely reported as missing because they lacked a dedicated
#[pyfunction] in Rust. The user-facing API is the Python layer, so
coverage should be measured there.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: timsaucer

.claude/skills/check-upstream/SKILL.md

@@ -8,6 +8,8 @@ argument-hint: [area] (e.g., "scalar functions", "aggregate functions", "window
 
 You are auditing the datafusion-python project to find features from the upstream Apache DataFusion Rust library that are **not yet exposed** in this Python binding project. Your goal is to identify gaps and, if asked, implement the missing bindings.
 
+**IMPORTANT: The Python API is the source of truth for coverage.** A function or method is considered "exposed" if it exists in the Python API (e.g., `python/datafusion/functions.py`), even if there is no corresponding entry in the Rust bindings. Many upstream functions are aliases of other functions — the Python layer can expose these aliases by calling a different underlying Rust binding. Do NOT report a function as missing if it appears in the Python `__all__` list and has a working implementation, regardless of whether a matching `#[pyfunction]` exists in Rust.
+
 ## Areas to Check
 
 The user may specify an area via `$ARGUMENTS`. If no area is specified or "all" is given, check all areas.
@@ -24,9 +26,9 @@ The user may specify an area via `$ARGUMENTS`. If no area is specified or "all"
 
 **How to check:**
 1. Fetch the upstream scalar function documentation page
-2. Compare against functions listed in `python/datafusion/functions.py` (check the `__all__` list)
-3. Also check `crates/core/src/functions.rs` for what's registered in `init_module()`
-4. Report functions that exist upstream but are missing from this project
+2. Compare against functions listed in `python/datafusion/functions.py` (check the `__all__` list and function definitions)
+3. A function is covered if it exists in the Python API — it does NOT need a dedicated Rust `#[pyfunction]`. Many functions are aliases that reuse another function's Rust binding.
+4. Only report functions that are missing from the Python `__all__` list / function definitions
 
 ### 2. Aggregate Functions
 
@@ -40,8 +42,9 @@ The user may specify an area via `$ARGUMENTS`. If no area is specified or "all"
 
 **How to check:**
 1. Fetch the upstream aggregate function documentation page
-2. Compare against aggregate functions in `python/datafusion/functions.py`
-3. Report missing aggregate functions
+2. Compare against aggregate functions in `python/datafusion/functions.py` (check `__all__` list and function definitions)
+3. A function is covered if it exists in the Python API, even if it aliases another function's Rust binding
+4. Report only functions missing from the Python API
 
 ### 3. Window Functions
 
@@ -55,8 +58,9 @@ The user may specify an area via `$ARGUMENTS`. If no area is specified or "all"
 
 **How to check:**
 1. Fetch the upstream window function documentation page
-2. Compare against window functions in `python/datafusion/functions.py`
-3. Report missing window functions
+2. Compare against window functions in `python/datafusion/functions.py` (check `__all__` list and function definitions)
+3. A function is covered if it exists in the Python API, even if it aliases another function's Rust binding
+4. Report only functions missing from the Python API
 
 ### 4. Table Functions
 
@@ -70,8 +74,9 @@ The user may specify an area via `$ARGUMENTS`. If no area is specified or "all"
 
 **How to check:**
 1. Fetch the upstream table function documentation
-2. Compare against what's available in this project
-3. Report missing table functions
+2. Compare against what's available in the Python API
+3. A function is covered if it exists in the Python API, even if it aliases another function's Rust binding
+4. Report only functions missing from the Python API
 
 ### 5. DataFrame Operations
 
@@ -84,9 +89,9 @@ The user may specify an area via `$ARGUMENTS`. If no area is specified or "all"
 
 **How to check:**
 1. Fetch the upstream DataFrame documentation page listing all methods
-2. Compare against methods in `python/datafusion/dataframe.py`
-3. Also check `crates/core/src/dataframe.rs` for what's implemented
-4. Report DataFrame methods that exist upstream but are missing
+2. Compare against methods in `python/datafusion/dataframe.py` — this is the source of truth for coverage
+3. The Rust bindings (`crates/core/src/dataframe.rs`) may be consulted for context, but a method is covered if it exists in the Python API
+4. Report only methods missing from the Python API
 
 ### 6. SessionContext Methods
 
@@ -99,9 +104,9 @@ The user may specify an area via `$ARGUMENTS`. If no area is specified or "all"
 
 **How to check:**
 1. Fetch the upstream SessionContext documentation page listing all methods
-2. Compare against methods in `python/datafusion/context.py`
-3. Also check `crates/core/src/context.rs` for what's implemented
-4. Report SessionContext methods that exist upstream but are missing
+2. Compare against methods in `python/datafusion/context.py` — this is the source of truth for coverage
+3. The Rust bindings (`crates/core/src/context.rs`) may be consulted for context, but a method is covered if it exists in the Python API
+4. Report only methods missing from the Python API
 
 ### 7. FFI Types (datafusion-ffi)