[engine] Implement Data Flow and Taint Analysis for precise variable tracking

[Gbangbolaoluwagbemiga]

Context

Currently, Sanctifier uses AST-based pattern matching. This fails if variables are passed through intermediate functions or aliased.

What to build

Implement an intra-procedural Control Flow Graph (CFG) and taint analysis pass.

1. Identify "sources" (untrusted user input).
2. Identify "sinks" (privileged operations).
3. Trace data flow between them.

Acceptance Criteria

• CFG generation module in sanctifier-core.
• Taint engine tracks variable assignments.