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

Cargo.toml

@@ -13,3 +13,7 @@ regex = "1"
 git2 = "0.19"
 walkdir = "2"
 anyhow = "1"
+semver = "1"
+
+[dev-dependencies]
+tempfile = "3"