fix: Extract mp-slug before property normalization

Fix bug where custom slugs (mp-slug) were being ignored because they
were extracted from normalized properties after being filtered out.

The root cause: normalize_properties() filters out all mp-* parameters
(line 139) because they're Micropub server extensions, not properties.
The old code tried to extract mp-slug from the normalized properties
dict, but it had already been removed.

The fix: Extract mp-slug directly from raw request data BEFORE calling
normalize_properties(). This preserves the custom slug through to
create_note().

Changes:
- Move mp-slug extraction to before property normalization (line 290-299)
- Handle both form-encoded (list) and JSON (string or list) formats
- Add comprehensive tests for custom slug with both request formats
- All 13 Micropub tests pass

Fixes the issue reported in production where Quill-specified slugs
were being replaced with auto-generated ones.

References:
- docs/reports/custom-slug-bug-diagnosis.md (architect's analysis)
- Micropub spec: mp-slug is a server extension parameter

Generated with Claude Code

Co-Authored-By: Claude <noreply@anthropic.com>

starpunk/micropub.py

@@ -287,6 +287,17 @@ def handle_create(data: dict, token_info: dict):
             "insufficient_scope", "Token lacks create scope", status_code=403
         )
 
+    # Extract mp-slug BEFORE normalizing properties (it's not a property!)
+    # mp-slug is a Micropub server extension parameter that gets filtered during normalization
+    custom_slug = None
+    if isinstance(data, dict) and 'mp-slug' in data:
+        # Handle both form-encoded (list) and JSON (could be string or list)
+        slug_value = data.get('mp-slug')
+        if isinstance(slug_value, list) and slug_value:
+            custom_slug = slug_value[0]
+        elif isinstance(slug_value, str):
+            custom_slug = slug_value
+
     # Normalize and extract properties
     try:
         properties = normalize_properties(data)
@@ -295,14 +306,6 @@ def handle_create(data: dict, token_info: dict):
         tags = extract_tags(properties)
         published_date = extract_published_date(properties)
 
-        # Extract custom slug if provided (Micropub extension)
-        custom_slug = None
-        if 'mp-slug' in properties:
-            # mp-slug is an array in Micropub format
-            slug_values = properties.get('mp-slug', [])
-            if slug_values and len(slug_values) > 0:
-                custom_slug = slug_values[0]
-
     except MicropubValidationError as e:
         raise e
     except Exception as e: