feat(core): Dependency Graph (#65)

Here’s a draft GitHub PR description you can drop into your merge
request:

This PR introduces a `torch.fx.Graph` style **doubly‐linked** dependency
graph
- Nodes wrap an opaque "statement" plus its input/output variables  
- Maintains:
    - a `stmt_to_node` map to find a node by its statement  
- a `var_to_producer` map from each variable to the node that produces
it
- a `var_to_consumers` map from each variable to all nodes that consume
it
- Provides safe, consistent APIs to:
    - create, insert (before/after), replace, erase nodes
    - query producers/consumers by node or by variable  

### Why it matters
- Enables IR-level transformations and analyses that need to track data
dependencies
- Guarantees correctness of producer/consumer relationships as the graph
is edited
- Lays the groundwork for scheduling passes (topological sort, liveness,
CSE, fusion, etc.)

### Unittests
50 pytest cases covering:
- basic chain graphs, branching graphs, empty/inputs‐only graphs  
- all API calls, success & failure modes (invalid anchors, duplicate
stmts/vars, head/leaf errors)
- identity vs. equality, iteration‐stop, idempotent clears, and more

Author: potatomashed

cpp/c_api.cc

@@ -1,3 +1,4 @@
+#include "./dep_graph.h"
 #include "./registry.h"
 #include <mlc/core/all.h>