Add analyze command to detect reachable vulnerable symbols in Rust projects

Implements the next phase of cargo-deep-audit: given a Rust project and the
enriched vulnerability database, determine which vulnerable symbols are
actually called from user code.

New modules:
- lockfile.rs: Parse Cargo.lock to extract dependency names and versions
- scanner.rs: Scan .rs source files for references to vulnerable symbols
  using import tracking (use statements) and call pattern matching, with
  High/Medium confidence levels
- analyzer.rs: Orchestrate the full pipeline — load vuln_db, match against
  lockfile versions (semver-aware), scan source, produce structured report

Changes:
- main.rs refactored into subcommands: `enrich` (Phase 1) and `analyze` (Phase 2)
- db.rs/diff_analyzer.rs: Added Deserialize support to load vuln_db.json
- CI workflow updated for subcommand syntax
- 24 tests total (21 unit + 3 integration), all passing

https://claude.ai/code/session_011sdj18ZvQYbDVwEKqrWDj1

tests/fixtures/test_project/Cargo.lock

@@ -0,0 +1,22 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "hyper"
+version = "0.14.27"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "tokio"
+version = "1.32.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "serde"
+version = "1.0.190"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "test-app"
+version = "0.1.0"