fix: set DEFAULT_COVERAGE to 0.05 (experimentally determined sweet spot)

The default min_coverage of 0.8 was filtering out almost all symbols
before CRX could see them, making every other parameter tuning moot.
At 0.8, only symbols present in 80%+ of methods survive — which means
0-2 symbols per group. At 0.05, 46 symbols survive for fastapi/src
and CRX can actually find structure.

This was the root cause of why max_unique_symbols gate never triggered
and why CRX kept producing flat bags.

bex/tag_preprocessor/analyze.py

@@ -173,7 +173,7 @@ def scan_directory(dir_path, gitignore_spec=None):
     return result
 
 
-DEFAULT_COVERAGE = 0.8
+DEFAULT_COVERAGE = 0.05
 
 VERBOSE = False
 _vstart = 0.0