Add syn-based type tracking and golden codebase integration tests

Type tracking (type_tracker.rs):
- Uses syn AST parsing to resolve variable types from function parameters,
  let bindings with annotations, constructor calls (Type::new()), struct
  literals, builder patterns, and closure parameters
- Integrated into scanner to promote method-call matches from Medium to
  High confidence when the receiver's type is known

Golden integration tests:
- vulnerable_project: 4 vulnerable deps, 3 with reachable calls across
  multiple files (qualified calls, typed method calls, constructor-inferred
  types). Verifies regex dep is listed but Compiler::compile is NOT reachable.
- safe_project: patched deps (hyper, smallvec) excluded, vulnerable deps
  (tokio, regex) listed but no symbols reachable. Reports POSSIBLY SAFE.
- clean_project: no vulnerable deps at all. Reports CLEAN.

37 tests total (33 unit + 4 integration), all passing.

https://claude.ai/code/session_011sdj18ZvQYbDVwEKqrWDj1

Cargo.toml

@@ -14,6 +14,7 @@ git2 = "0.19"
 walkdir = "2"
 anyhow = "1"
 semver = "1"
+syn = { version = "2", features = ["full", "visit"] }
 
 [dev-dependencies]
 tempfile = "3"