feat: Add CPU features detection and stretched grid API (Phase 6)

MIGRATION_PLAN.md

@@ -430,26 +430,32 @@ cpu_features_t cfd_get_cpu_features(void);
 
 **Actual effort:** 0.5 days
 
-### Phase 6: Add CPU Features & Misc (Enhancement)
+### Phase 6: Add CPU Features & Misc (Enhancement) ✅ COMPLETED
 
 **Priority:** P2 - Nice to have
 
+**Status:** Completed on 2026-01-02
+
 **Tasks:**
 
-- [ ] **6.1 CPU features detection**
-  - `get_cpu_features()` → set of feature flags
-  - `has_avx2()`, `has_neon()` helpers
+- [x] **6.1 CPU features detection**
+  - Added `SIMD_NONE`, `SIMD_AVX2`, `SIMD_NEON` constants
+  - `get_simd_arch()` → returns SIMD architecture constant
+  - `get_simd_name()` → returns "avx2", "neon", or "none"
+  - `has_avx2()` → bool (checks AVX2 availability with OS support verification)
+  - `has_neon()` → bool (checks ARM NEON availability)
+  - `has_simd()` → bool (checks if any SIMD is available)
 
-- [ ] **6.2 Grid initialization variants**
-  - `grid_uniform(nx, ny, ...)`
-  - `grid_chebyshev(nx, ny, ...)`
-  - `grid_geometric(nx, ny, ...)`
+- [x] **6.2 Grid initialization variants**
+  - `create_grid_stretched(nx, ny, xmin, xmax, ymin, ymax, beta)` - Hyperbolic cosine stretching
+  - Clusters points toward the domain center (symmetric stretching)
+  - Note: Chebyshev and geometric variants not available in C library
 
-- [ ] **6.3 Update solver list**
-  - Add new OMP solver types
-  - Update documentation
+- [x] **6.3 Add tests**
+  - Created `tests/test_cpu_features.py` with 26 tests
+  - Tests for SIMD constants, detection functions, and grid stretching
 
-**Estimated effort:** 1 day
+**Actual effort:** < 0.5 days
 
 ### Phase 7: Documentation & Tests (Required)
 
@@ -562,10 +568,10 @@ find_library(CFD_LIBRARY cfd_library)  # Unified library name
 | Phase 3: Derived Fields | ~~1-2 days~~ ✅ < 1 day | 3.5 days |
 | Phase 4: Error Handling | ~~1 day~~ ✅ < 0.5 days | 4 days |
 | Phase 5: Backend Availability (v0.1.6) | ✅ 0.5 days | 4.5 days |
-| Phase 6: CPU Features | 1 day | 5.5 days |
-| Phase 7: Docs & Tests | 2 days | 7.5 days |
+| Phase 6: CPU Features | ~~1 day~~ ✅ < 0.5 days | 5 days |
+| Phase 7: Docs & Tests | 2 days | 7 days |
 
-**Total estimated effort:** ~~9-10 days~~ ~7.5 days (4.5 days completed)
+**Total estimated effort:** ~~9-10 days~~ ~7 days (5 days completed)
 
 ---
 
@@ -590,3 +596,101 @@ find_library(CFD_LIBRARY cfd_library)  # Unified library name
 6. Correctly detects available backends at runtime
 7. Python API is Pythonic and well-documented
 8. Examples run successfully
+
+---
+
+## Pythonic API Assessment
+
+**Assessment Date:** 2026-01-02
+
+### ✅ What's Pythonic (Good Practices)
+
+**1. Exception Hierarchy Design** - Excellent
+- Multiple inheritance with standard Python exceptions (`CFDMemoryError(CFDError, MemoryError)`)
+- Enables idiomatic exception handling: `except MemoryError` catches CFD memory errors
+- The `raise_for_status()` pattern follows requests library conventions
+- Docstrings with attribute documentation
+
+**2. Naming Conventions** - Good
+- Functions use `snake_case` (`create_grid`, `run_simulation`, `has_avx2`)
+- Constants use `UPPER_CASE` (`SIMD_AVX2`, `BC_TYPE_DIRICHLET`)
+- Private modules prefixed with underscore (`_loader.py`, `_exceptions.py`)
+
+**3. Return Types** - Good
+- Functions return native Python types (dict, list, bool, int) instead of custom objects
+- `create_grid()` returns a dict with intuitive keys (`nx`, `ny`, `x_coords`)
+- `run_simulation_with_params()` returns structured dict with `stats` sub-dict
+
+**4. Module Organization** - Good
+- Clean `__all__` exports
+- Graceful handling of unbuilt extension (`ExtensionNotBuiltError`)
+- Dynamic solver constants discovery
+
+### ⚠️ Areas for Improvement
+
+**1. ~~Inconsistent Coordinate Naming~~ (FIXED)**
+```python
+# Both functions now return consistent keys:
+grid["x_coords"], grid["y_coords"]  # standardized naming
+```
+
+**2. Mixed Paradigms for Complex Operations**
+```python
+# Positional args (C-style):
+create_grid(10, 10, 0.0, 1.0, 0.0, 1.0)
+
+# More Pythonic alternative:
+create_grid(nx=10, ny=10, bounds=(0.0, 1.0, 0.0, 1.0))
+# or
+create_grid(nx=10, ny=10, xmin=0.0, xmax=1.0, ymin=0.0, ymax=1.0)
+```
+
+**3. Verbose Boundary Condition API**
+The BC functions require separate calls for each boundary type. A more Pythonic approach might be:
+```python
+# Current:
+bc_apply_inlet_uniform(u, v, nx, ny, u_inlet, v_inlet, BC_EDGE_LEFT)
+bc_apply_outlet_velocity(u, v, nx, ny, BC_EDGE_RIGHT)
+
+# More Pythonic (context manager or builder pattern):
+with BoundaryConditions(u, v, nx, ny) as bc:
+    bc.left.inlet_uniform(u=1.0, v=0.0)
+    bc.right.outlet()
+```
+
+**4. Constants Could Use IntEnum**
+```python
+# Current:
+SIMD_NONE = 0
+SIMD_AVX2 = 1
+
+# More Pythonic:
+class SIMDArch(IntEnum):
+    NONE = 0
+    AVX2 = 1
+    NEON = 2
+```
+This provides `str(SIMDArch.AVX2)` → `"SIMDArch.AVX2"` and better IDE support.
+
+**5. No Type Hints in Public API**
+While `_exceptions.py` has type hints, the C extension functions lack type stubs (`.pyi` files) for IDE autocompletion.
+
+### 📊 Summary
+
+| Aspect | Rating | Notes |
+|--------|--------|-------|
+| Naming | ★★★★☆ | Follows PEP 8, minor inconsistencies |
+| Error Handling | ★★★★★ | Excellent exception hierarchy |
+| Return Types | ★★★★☆ | Native types, dict structure could use dataclasses |
+| API Design | ★★★☆☆ | Functional but verbose, mirrors C API closely |
+| Type Annotations | ★★☆☆☆ | Missing .pyi stubs for C extension |
+| Documentation | ★★★★☆ | Good docstrings in `__init__.py` |
+
+**Overall:** The code is reasonably Pythonic for a C extension binding. The exception handling is particularly well-designed. The main improvements would be adding type stubs and potentially providing higher-level Pythonic wrappers around the lower-level C-style functions.
+
+### Recommended Future Enhancements
+
+1. **Add type stubs** (`cfd_python.pyi`) for IDE autocompletion
+2. **Standardize coordinate naming** across grid functions
+3. **Consider IntEnum** for constant groups (SIMD, BC types, backends)
+4. **Optional high-level wrappers** for BC operations (Phase 8 candidate)