Implements resolve_type3() for Type 3 font encoding resolution using
the Type 3-specific fallback chain:
- L1: ToUnicode CMap (confidence 1.0)
- L2: Encoding + AGL (confidence 0.9)
- L3: SKIPPED (no embedded program for Type 3)
- L4: Shape recognition (confidence 0.7)
Adds ShapeEntry, ShapeMatch types and lookup_shape() stub function.
Fixes overflow bug in Type3Font::load_widths().
Closes: pdftract-1uj5
