L2 does not resolve call sites inside closures #17

Describe the bug
L2 never resolves call sites inside closures (function literals). CallGraphBuilder.Build (internal/semantic_analysis/call_graph.go) walks top-level functions and methods only, not InnerCallables, so every call node inside a closure keeps callee: null and produces no call_graph edge. On testdata/chi, all 245 closure call nodes are unresolved. This predates the L3 branch (main behaves the same).

To Reproduce

1. cango -i testdata/chi -a 2 -o /tmp/out
2. Find any callable under another callable's callables{} (a closure_N) and inspect its body call nodes.
3. Every callee is null, and no call_graph edge has a closure as src.

Expected behavior
Call sites inside closures resolve like any other call site: callee is the callee's id (in-project or @external), and call_graph has an edge from the closure's callable id.

Logs
Not applicable: the output is valid; the facts are missing.

Additional context

• Fix needs closures to carry their declaring file (Path) so the dedup key (Path, Signature, callee) and emission's (Signature, SourcePath) resolution keep working.
• Matters before L4: interprocedural edges out of closures (handlers, callbacks, goroutines) depend on it.
• Found by the whole-branch review of PR Go L3 intraprocedural dataflow, app-outermost ids and L2 library calls #16.