Add expression-evaluator: DAGs & state machines tutorial project

Educational calculator teaching FSMs (explicit transition table tokenizer)
and DAGs (recursive descent parser with AST evaluation). Includes CLI with
REPL, graphviz visualization, and 61 tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

python/expression-evaluator/CLAUDE.md

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+# Expression Evaluator
+
+## Overview
+Educational project teaching DAGs and state machines through a calculator.
+Pure Python, no external dependencies.
+
+## Running
+```bash
+python main.py "3 + 4 * 2"                          # single expression
+python main.py                                        # REPL mode
+python main.py --show-tokens --show-ast --trace "expr" # show internals
+python main.py --dot "3+4*2" | dot -Tpng -o ast.png   # AST diagram
+python main.py --dot-fsm | dot -Tpng -o fsm.png       # FSM diagram
+```
+
+## Testing
+```bash
+python -m pytest tests/ -v
+```
+
+## Architecture
+- `tokenizer.py` -- Explicit finite state machine (Mealy machine) tokenizer
+- `parser.py` -- Recursive descent parser building an AST (DAG)
+- `evaluator.py` -- Post-order tree walker (topological sort evaluation)
+- `visualize.py` -- Graphviz dot generation for AST and FSM diagrams
+- `main.py` -- CLI entry point with argparse, REPL mode
+
+## Key Design Decisions
+- State machine uses an explicit transition table (dict), not implicit if/else
+- Unary minus resolved by examining previous token context
+- Power operator (`^`) is right-associative (grammar uses right-recursion)
+- AST nodes are dataclasses; evaluation uses structural pattern matching
+- Graphviz output is raw dot strings (no graphviz Python package needed)
+
+## Grammar
+```
+expression ::= term ((PLUS | MINUS) term)*
+term       ::= unary ((MULTIPLY | DIVIDE) unary)*
+unary      ::= UNARY_MINUS unary | power
+power      ::= atom (POWER power)?
+atom       ::= NUMBER | LPAREN expression RPAREN
+```