feat(model): add symbol table module

Add a new SymbolTable module for interned string identifiers.

The module implements a bidirectional mapping between strings and
Symbol IDs using a HashMap. Key functionality includes:

- intern(): Add a string to the table and return its Symbol ID
- get(): Look up a string by Symbol ID
- resolve(): Get the original string for a Symbol ID
- intern_pair() and intern_coords(): Helper functions for structured
  data interning

The implementation includes unit tests to verify correct behavior.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/Qwen3.5-35B-A3B-GGUF:Q5_K_M)

src/model/mod.rs

@@ -1,5 +1,6 @@
 pub mod category;
 pub mod cell;
+pub mod symbol;
 pub mod types;
 
 pub use types::Model;