Small correctness and test-coverage gaps from the L3 reviews #28

Small, independent items found by the reviews of PR #16. None hides a known high-impact bug. Pick off one at a time and tick it here.

Correctness

• type I2 I1 (a defined type over an interface) gets kind: interface but no methods or base types. Populate its method set from the underlying interface.

• Panic detection misses a method value taken through a struct that embeds an interface (s.M, where s embeds a nil interface). No exception edge is added.

• Unresolved receiver types. A method value on a receiver whose type failed to type-check is recorded as a field path (req.Set) in ddg var. Record it as a method use. This one is cosmetic.

• Builtin-scope types. Calls on them, such as err.Error(), get a junk internal callee signature (.error.Error). The emitted callee is correctly null. Record no signature.

• Symlinked project directories. Library detection compares paths with filepath.Abs but not EvalSymlinks, so a symlinked project directory can misclassify files. Resolve symlinks before comparing.

• Spans in malformed functions. In a function with severe syntax errors, L3 body-node spans can be inverted or point into another file (e.g. bytes [113,0]). There is no crash. BuildNodes takes End positions from fset.Position without checking they are in the node's file (internal/dataflow/bodynodes.go). Clamp End to Pos's file.

• Reassigning a slice from append kills its element defs. In s[0] = 5; s = append(s, n); return s[0], the strong def of s kills the earlier s[*] def, although append may return the same backing array. Treat s = append(s, …) as a weak def of s[*].

Test coverage

• Superset check. TestGateL2_SupersetOfL1 builds its L2 input without external_symbols, so it skips the library-edge path.
• Satellite call nodes. Their parent assignment is untested for calls in folded for init/post, case lists, select comm clauses and range expressions. Do this before L4: port wiring depends on it.
• Range forms. Add a Go 1.23+ fixture module with exact-edge CFG tests for range over an integer and over an iterator function. testdata/dataflow declares go 1.21.

Consolidates #24 and #25.