Small L1/L3 correctness gaps (defined interfaces, embedded-interface panics, builtin callees, symlinks) #24

Describe the bug
Several small correctness gaps, each rare, found by the reviews of PR #16:

1. type I2 I1 (a defined type over an interface) gets kind: interface but no methods or base types; its method set is not populated from the underlying interface.
2. Panic detection misses a method value taken through a struct that embeds an interface (s.M where s embeds a nil interface), so no exception edge is added.
3. A method value on a receiver whose type failed to type-check is recorded as a field path (req.Set) in ddg var. Cosmetic.
4. Calls on builtin-scope types, such as err.Error(), get a junk internal callee signature (.error.Error). The emitted callee is correctly null, but the internal value is wrong.
5. Library detection compares paths with filepath.Abs but not EvalSymlinks, so a symlinked project directory can misclassify files.

To Reproduce
Each item names its construct. Fixtures for 1–4 fit in testdata/dataflow.

Expected behavior

1. Methods and base types populated.
2. An exception edge.
3. A method use, not a field.
4. No signature recorded.
5. Symlinks resolved before comparing.

Logs
Not applicable.

Additional context
Low priority; grouped to avoid five tiny issues. Split any that grows.